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Content last updated: November 21, 2023

Third Party Interventions Explained: What are they?

This Toolkit does not constitute legal advice.

If you are considering legal action in the form of a third party intervention, we recommend that you speak with a solicitor. If you are unable to retain a solicitor, and if you are a PILS member, you can submit an application for legal support.

What is a third party intervention?

A third party intervention is when an individual or organisation who is not a party to existing proceedings applies to participate in those proceedings. The term “third party” relates to the position that the intervening individual or group holds in the eyes of the court. A party becomes a third party intervener when the court accepts their application to participate in the proceedings. There are a number of different ways that a third party can participate in judicial proceedings:

An individual or group may intervene in litigation informally by providing documents to support one party in proceedings. This could include collaborating with a party on legal arguments or evidence to strengthen the case. Alternatively, an individual or group could support the litigation by providing a formal witness statement or expert report to a party to the litigation. An organisation does not need permission from the court to support a party to litigation in this way.

The bulk of this section will focus, however, on formal third party interventions, where the court grants an individual or a group permission to become a party to the proceedings. Becoming a party to the proceedings enables the individual or group to provide written and/or oral representation to the court.

Each of these interventions is taken with the express aim of supporting one of the parties in the litigation. A third party will intervene to strengthen one party’s position by providing additional information which it thinks will help to persuade the court in one party’s favour.

A third party intervention is distinct from a process called “amicus curiae”, which means “friend of the court”. An amicus curiae is there to assist the court, not a particular party. PILS has previously produced a guide on amicus curiae. Consult this guide if you’re interested in whether your organisation might be suited to act as an amicus curiae.

The rest of this section of the toolkit will focus specifically on how to plan and execute effective formal third-party interventions.

How will I know about cases?

Before you can assess whether it would be appropriate for your organisation to intervene in proceedings, you have to know about those proceedings. Unfortunately, there is no central database in Northern Ireland that holds information about upcoming proceedings. In practice, organisations who take third party interventions are often approached by a party to the proceedings who will ask them to intervene to advance arguments that fall within that organisation’s remit or expertise. Your organisation can also learn about proceedings through media outlets, mailing lists, and coalition groups.

To Intervene or Not to Intervene

The decision as to whether you will be able to intervene in proceedings is ultimately up to the court. You must obtain leave from the court to intervene in judicial proceedings. Whether the court will grant leave to intervene is always a matter for the court’s discretion.

Before we address how to request the court to exercise its discretion to allow a third party intervention, we will discuss who can request leave to intervene and key factors that your organisation should consider before applying to intervene in an ongoing matter.

Will my organisation’s intervention assist the court?

This is the most important question to ask yourself if your organisation is considering applying to become a third party intervenor in proceedings. The purpose of making a third party intervention is to help the court come to the right conclusion. A party’s motivation to intervene in proceedings must be to put forward particular arguments or information that it thinks the court should consider. Whether the court will grant a third party leave (permission) to intervene will usually depend on whether the intervention will help the court decide the matter. The court will balance the benefits that the intervention brings against the inconvenience, delay and expense that a third party intervention can cause to the existing party. It is therefore important that you demonstrate that your intervention will help court to reach its decision.

A third party cannot and should not merely repeat points the other parties have raised. The third party seeking to intervene must add something new. Case law has said that an intervention does not assist the court where it simply repeats points raised by one of the parties to the proceedings.


A third party intervention must add something new.

This is the golden rule in third party litigations and should be the determinising factor as to whether your organisation applies to intervene in proceedings.

Can my organisation intervene?

Organisations of all kinds, including charities, non-governmental organisations, statutory bodies, and government departments can be suitable candidates to intervene in proceedings. The nature or structure of an organisation does not matter as much as the quality of the group’s contribution to the proceedings.

The content of your third party intervention must contribute something new to the proceedings that your organisation is uniquely positioned to provide. An application to intervene is more likely to be successful where the proposed intervener is able to provide relevant information that, without their intervention, would not be presented to the court. Organisations that represent the interests of a particular group who might be impacted by the case, or organisations that might have access to relevant information or have an expertise in the subject matter that the case addresses would be well-suited to intervene in proceedings.

Case Study: Friends of the Earth NI’s Intervention in the No Gas Caverns Case

Photo credit: PILS

Friends of the Earth applied to intervene on the basis that the legal challenge is strategically significant to protecting the environment in Northern Ireland, and that they, as a grassroots environmental campaigning charity with the overarching strategic objective of defending and expending environmental rights and protection, were well placed to bring this challenge. In their application to intervene, they advised that they supported the concerns that the applicant raised, and also wanted to provide submissions on two additional issues to which they were well suited to address: concerns around the Department’s consultation process in granting mineral licenses and the Department’s approach of considering Habitats Regulation Assessments as standalone projects (thereby overlooking the cumulative environmental impact of projects which require mineral licenses). They shared the applicant’s concern that the relevant licenses expose the environment to significant damage and wanted to use their voices to support both the DSDC, and highlight the significant public interest issues that the challenge raises.

Friends of the Earth was granted permission to intervene in 2021. Friends of the Earth has submitted their written submissions, which were accepted by the Court. This is a victory because Friends of the Earth’s involvement ensures that issues in these public interest issues, which were not addressed by the submissions of the parties to the proceedings, are put before the court. This case is still ongoing.

Should My Organisation Intervene?

Is intervention appropriate in this case?

An organisation interested in intervening should first satisfy itself of two things: (1) that the relevant proceedings raise important public interest issues; and, (2) that the public interest issue may not be sufficiently addressed by the parties already involved in the proceedings. It is generally not appropriate to intervene in a matter unless these two factors are present.

To determine whether the litigation raises a public interest issue, consult the Introduction to Public Interest Litigation section of this toolkit.

The best way to determine whether the parties to the litigation can sufficiently address the public interest issue is by asking one of the parties to the proceedings for a copy of the papers. Remember, if you are considering a third party intervention, you will submit arguments that will support the court’s determination in favour of one party. That party will likely be grateful for your interest in supporting the matter and will usually be happy to share the papers with you.

Is my organisation well placed to intervene?

A third party intervener must be able to address the public interest issues raised in the litigation in way that a party to the proceedings cannot. There are a number of different ways that your organisation may be uniquely positioned to assist the court and add value to a case beyond what a party to the proceedings has already provided. Think about what your organisation does well and how that work can help the court decide the relevant case.

Are you working on the ground in a grassroots organisation? Your organisation may be able to provide the court with evidence of the direct impact of a particular policy or legislative provision on individuals in your community.

Does your organisation engage in advocacy in a particular area? Your organisation may have knowledge on policy aims and legislative history that could assist the court in interpreting a particular statute or provision.

Are other parties intervening?

If particular proceedings may impact an area that is relevant to the work of a number of organisations, you could consider taking part in, or coordinating, a joint intervention. Joint interventions work most effectively where there is common ground between the various groups interested in intervening in the proceedings. If you are considering a joint intervention, it is important to ensure that you have the same objectives and intend to advance the same interests.

Joint interventions can be incredibly successful because they can carry more weight, and make a more significant and fulsome contribution to a case. Courts may also be more likely to grant leave to a joint intervention because it will reduce delays and costs.

Does the intervention advance your organisation’s goals?

While interventions are less work than being the applicant in proceedings, a successful intervention will still require a significant commitment of time and resources. It is important to make sure that this case is of significant importance to your organisation before requesting leave to intervene in proceedings.

Consider whether this matter is likely to impact the people who use your services or who you support in your work. Consider how important this case might be to your organisational objectives, or whether its determination, one way or the other, would mark a significant benefit or hinderance to your objectives. Consider whether this case might significantly change an area of law that is important to your work.

Where the potential impact to your organisation of a particular legal decision outweighs the cost to your organisation of bringing an intervention, intervening may be the right path.

Is my organisation ready for the increased publicity?

Many of the public interest cases that permit interveners also attract attention from the wider public and the media. It is important that you consider and plan how you will deal with this publicity. This might require you to devise an early public relations strategy, draft press releases and prepare your staff or trustees for interviews.

It is also important to consider the reputational risks of intervening. Unfortunately, unsuccessful interventions can result in the intervening organisation being associated with bringing about an unwelcomed change in the law. You want to be confident that, no matter the outcome, your organisation did the right thing by trying to influence the outcome of this decision. Consider whether the risk of you failing to intervene outweighs those potential reputational risks of a failed intervention.

Considering Intervening?

You can learn about how to make a third party intervention in Third Party Interventions: When, How and How Much?