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Content last updated: November 21, 2023

Third Party Intervention Basics: How, When, and How Much?

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How to Intervene

An organisation must file a letter with the court to request permission to intervene in particular proceedings. While this is the formal process that the court directs, there are practical steps that your organisation should take before writing to the court.

You may want to seek legal assistance or representation to support your organisation with its third party intervention. PILS can support your organisation with its third party intervention by recruiting legal support from barristers and solicitors on our Pro Bono Register.

Letter to the Parties

The practical first step for an organisation wishing to intervene in proceedings is to contact the parties to those proceedings. The court will want to know whether the parties to the proceedings support your intervention. You therefore need to ask the other parties if they would support your application to intervene before you lodge that application with the court. This is an essential first step because the court will likely refuse your application if you have not contacted the other parties to the relevant proceedings.

Your letter to the other parties should set out:

  • Who you are – explain the focus of your organisation’s work
  • Why you want to intervene – explain your organisation’s interest in the subject matter or the impact that subject matter has on your organisation’s work
  • What you want to say in your intervention – explain the law, evidence, and arguments that you intend to submit if you are granted leave to intervene

It is common for the other parties in the proceedings to refuse to consent to your application. Some parties may be concerned that an intervention will make the proceedings more complex and lengthier. Some parties may, alternatively, advise you that they will not take any position on your application because, in their view, it is a matter for the court to decide. Neither of these responses are fatal to your application to intervene. The court will grant you leave to intervene if it thinks that your intervention will assist them and add value to the proceedings.

Letter to the Court

Applying to be a third party intervener requires filing a letter with the court. There is no prescribed form, but the court has provided clear guidance on what information must be included in that letter:

1.     The name and contact details of the individual or organisation seeking third party intervener status;

2.     The nature of that individual or organisation’s interest in the proceedings;

In this section, you should provide a description of your organisation, including its public policy aims, activities, membership and or/people it represents. You need to show what value you are able to add to the proceedings, and so you should also provide an outline of your specialist knowledge or expertise relevant to the issues raised.

3.     An indication of the content of the proposed intervention and how allowing the intervention would promote the interests of justice;

This is where you need to set out the scope of your intervention and what your organisation’s intervention will address. Provide a statement of the public interest issues that this case raises, their impact on the public generally or sectors of it, and the arguments that you want to advance. Cross-referencing your submissions with the parties’ skeleton arguments or the proceedings’ initiating documents may be very helpful. This will help the court determine the grounds to which your proposed submissions refer and whether those interventions will assist its decision-making.

4.     The proposed means of intervention;

An intervenor can contribute to proceedings by providing written submissions, oral submissions, or both. You need to advise the court of the way in which you wish to intervene and the length, either in court time or number of pages, that you would require to make your intervention.

5.     Whether or not the parties to the proceedings have consented to the application;

As discussed above, you should have contacted the other parties to the proceedings before applying for leave to intervene with the court. You should advise the court of the response that you received from the other parties and to include the correspondence that you received from the other parties in your application.

6.     Details of any previous application(s) that you or your organisation made for leave to intervene in the proceedings, including the outcome;

7.     Confirmation of whether you or your organisation intend to make an application for a protective costs order if your application is successful;

The general rule in third party interventions is that interveners will obtain a Protective Costs Order (PCO) which provides that the intervener will pay their own costs. To learn more about PCO’s in third party interventions, you can either continue reading, skip ahead to the section that addresses costs in third party interventions, or jump to the section on PCO’s in third party interventions. You can also consult the PCO section of this toolkit for more general information on PCO’s.

8.     Any other relevant information.

Sending the Letter to the Court

This letter may be filed in the Court Office by way of email, post, facsimile or delivered by hand. It is the applicant’s responsibility to ensure that the court office has safely received your application. The court has a number of different offices. It is important to make sure that you file your letter in the appropriate office. If you are not sure in which court office you should file your letter, you can inquire with the Central Office at

You must also send a copy of your application to the other parties in the proceedings. As a general rule, all correspondence that you send to the court or to one party should be sent to all parties to the proceedings: the applicant, the respondent(s), any other third parties, and the court.

The Court’s Decision

The court will usually issue its decision on an application for third party intervention status within 2 working days of receiving the application. The court will typically communicate its decision in writing. In some instances, the court may convene an oral hearing to decide the application for a third party intervention. This, however, is rare, and entirely up to whether the court thinks that a hearing is necessary for it to make a decision. You should therefore assure yourself that all of the information that you want the court to consider is included in your letter to the court.

The court’s decision will advise you whether you have been granted intervener status. If the application is successful, the court will also set out the extent to which that party is permitted to intervene. The court may limit the scope of a party’s intervention to a particular issue or issues. It may grant the intervener leave to make written submissions only, to make written and oral submissions, or it may permit the intervener leave to make written submissions and advise that it will later determine whether it will grant the intervener leave to make oral submissions. Permission to make oral submissions is only likely to be granted in certain cases. If the court grants leave to make oral submissions, it will also set a time limit on those oral submissions.


The court has specific deadlines for third party interventions. However, these deadlines should be treated as minimum requirements. A third party intervention is always subject to the court’s discretion. You are therefore advised, where possible, to follow principles of best practice to increase the likelihood that the court allows your application for leave to intervene.

Court Deadlines

A letter requesting third party intervenor status must be filed with the court at least 21 working days before the hearing date and sent to all parties in the proceedings.

Where the court grants leave to intervene by way of written submissions, those submissions must be filed in the court office at least 13 days before the hearing date.

Principles of Best Practice

The court deadlines provide the minimum requirements for a successful application to intervene in proceedings. Generally, a party interested in intervening should begin the process as early as possible in the proceedings.

The rule is that any application to intervene should be made as early as is practical because permission to intervene may be refused on the basis of delay. The later in the proceedings that a person or organisation pursues intervention, the less likely other parties will be to agree to the intervention, and the less likely that the benefits of intervention will outweigh the increased delay, costs, and complication of the proceedings. An intervenor should also try to interfere as little as possible with the timetable leading up to and including the hearing itself.

When to Intervene

At what stage of the proceedings should I intervene?

Your organisation can intervene in judicial review proceedings either before or after leave has been granted. The general rule is that it is always best to intervene as early as possible. Intervening pre-leave is beneficial because you may be able to collaborate with the party that your intervention will support to ensure that there are grounds on which your organisation can provide added value through an intervention. Intervening post-leave can also be beneficial because you will know that the proceedings are going ahead before investing a great deal of time and energy into your intervention.

At what level of court should I intervene?

You can intervene in proceedings at a higher level of court if you didn’t intervene in the proceedings at a lower level of court. If you are seeking leave to intervene in these circumstances, it is best practice to advise the court as to why you did not participate in proceedings at the lower court and why you are now seeking to intervene on appeal. If you are seeking to intervene on appeal and did not intervene in earlier proceedings, consider whether the value that you want to add to the proceedings is relevant to the appeal proceedings. For example, any evidence to assist factual arguments would be better suited to interventions at the High Court, but arguments in which an organisation presents specialised legal knowledge might be suitable on appeal.

Waiting to intervene until a matter reaches a higher level of court is not always advisable. While those cases are more likely to have more of an impact and may attract media attention which could further your organisation’s profile, not every case is destined for the Supreme Court. If you believe that your intervention could help to facilitate a proper determination of a matter, getting involved in the lower proceedings is the only guaranteed way to ensure that your organisation can contribute to the judicial process.


Costs are always ordered at the court’s discretion. However, the general rule for cost orders in respect of third party interventions is that intervenors bear their own costs. This means that cost orders are not ordinarily made either in favour or against interveners. Any costs that other parties incur as a result of an intervention are costs in the proceedings which the party who is unsuccessful is likely to become liable for at the end of the matter.

The exception to this rule is that the court may order costs against an intervener where the court considers it just to do so. In most instances, this only occurs where an intervener conducts itself improperly. Costs may be ordered against an intervenor where they, in substance, act like a party to the proceedings, or their impropriety causes delay or results in the parties to the proceedings incurring additional costs.

What are my costs?

Costs that you might incur to bring a third party intervention will depend on the nature of your organisation and the kind of intervention that you want to make. There may be more costs involved where your organisation does not have legal expertise and requires representation to intervene. If your organisation employs a solicitor, your costs are likely to include staff time allocated for the work on the case, printing, and lodging fees. In either instance, if you are a PILS member, you can apply for support. If you require legal support, PILS can connect you with a solicitor or a barrister to support your intervention. You may also apply to PILS for financial support from PILS.  This support is available, subject to approval, as long as the underlying case is in the public interest.

How can I limit my cost exposure?

There is always some element of risk in legal proceedings. You can limit that risk by making cost agreements with the other parties or asking the court to grant a Protective Costs Order. You can also limit your exposure by ensuring that your conduct is appropriate and that your intervention disrupts the proceedings as little as possible.

Cost Agreements & Protective Costs Orders

The first practical step that you can take to limit your cost exposure is to reach an agreement with the other parties to the proceedings that you will not seek costs against one another.

If you’re not able to reach an agreement with the parties, you can ask that the court make a Protective Costs Order insulating you from any cost liability. The Costs section of this toolkit sets out the details on how to request a Protective Costs Order.

Conduct & Scope of Intervention

Where a third party intervener conducts themselves appropriately, the risk of a cost order being made against that intervener is very low. Conducting yourself appropriately in the context of a third party intervention requires the intervener to:

  • Communicate the proposed scope of your intervention as clearly as possible  the application for intervener status.
  • Only participate in the proceedings to the extent that you think your participation is essential to assist the court. If you can effectively communicate your points through written submissions, do not ask for permission to make oral submissions. If you can communicate everything to the court with two articles and ten pages of written submissions, do not include more. The greater your involvement increases without benefit to the court, the more likely an adverse cost order may be made against you.
  • Complying with the limits that the court has imposed on your intervention, or asking the court to amend its permission if appropriate. In any event, do not exceed the limits that the court has imposed on your intervention.
  • Display exemplary conduct in your communications with and towards all of the parties to the proceedings and the court. It is important to observe all relevant deadlines, follow court rules, ensure that all parties are properly served with court documents and included in correspondence with the court.

Summary of Advantages & Disadvantages of Third Party Interventions

Advantages Disadvantages
Cost effective Limited role
Time effective Little control over the case
Effective forum to raise public interest Limited cost risks