Amnesty International did it in the Pinochet cases – with a somewhat unexpected result. The United Synagogue did it in the Jewish Free School case. Secretaries of state do it regularly; the attorney general occasionally. Justice does it about four or five times a year. Overall, someone did it in about one-fifth of all the cases heard by the House of Lords between 2005 and 2009 (in 68 of 310 decisions). Third-party interventions are becoming an accepted part of the judicial process.

A third party to litigation is someone without a material interest in the litigation who is not acting as a friend of the court or amicus curiae. Historically, such intervenors were rare in the UK courts – though the Anti-Slavery Society had used equivalent procedures with some effect in its challenges to slavery in the 18th century. They were much more common in the US, where the practice developed of allowing the participation in Supreme Court hearings of someone who ‘was not a supplementary counsel for plaintiff or defendant, but a man sufficiently zealous for truth and justice to offer an unsolicited guidance to the court directly’.

Justice has recently published To Assist the Court: Third Party Interventions in the UK to analyse the current state of play. National bodies, such as the Equal Opportunities Commission and the Commission for Racial Equality, began intervening in cases from the late 1970s. It took until the mid-1990s for the first non-governmental organisation (NGO) to intervene successfully in a House of Lords case. Interventions are now acknowledged in, and governed by, both the Civil Procedure Rules and the Supreme Court Rules (SCR). But along with recognition has come cost. The SCR slaps on a hefty fee of £800 for the privilege – though this may be reduced or remitted for a charitable or not-for-profit organisation.

As experience of interventions has developed, oral representation has become more of the norm rather than simply relying on written submissions. As a result, NGOs have needed increasing assistance from the legal profession – both solicitors and barristers – to cope with the significant workload. Happily, many of the leading counsel are interested enough in the issues to give their time freely. There is rarely a problem in getting counsel of appropriate seniority for a major case and a number of leading QCs have generously – and willingly – appeared for Justice. Law firms have also been willing to devote resources on a pro bono basis. Justice has, for example, received magnificent support from a wide range of firms, including Freshfields, Bhatt Murphy, Clifford Chance, Dechert, Mayer Brown and Herbert Smith (which funded our report). All merit our profound thanks.

The attraction of an intervention to a lawyer is clear: you can concentrate on the law; the point is important. Compared with commercial litigation, the hearings – and cases – are short, sharp and focused. There are dangers, however. Lord Hoffmann sent the Northern Ireland Human Rights Commission away with a flea its ear: ‘I am bound to say that in this appeal the oral submissions on [its] behalf only repeated in rather more emphatic terms the points which had already been quite adequately argued by counsel for the appellant. In future, I hope that intervenors will avoid unnecessarily taking up the time of the court in this way.’

A degree of restraint is thus demanded of those who intervene. If they are really unlucky, they may suffer more than a lashing from a judicial tongue. The United Synagogue got landed with costs in the Court of Appeal. The secretary of state tried unsuccessfully to hide behind a local authority when he intervened in R (Barker) v London Borough of Bromley. Lord Hope ruled that ‘in my opinion the effect of this intervention was that he became a party to the proceedings’. For that reason prudent intervenors, such as Justice, seek the consent of the parties to intervene and an undertaking not to ask for costs. The crucial issue is that the intervenor has something to add to the litigation. In many of Justice’s interventions – which are often made jointly with other bodies – the additional element is often a detailed analysis of international comparisons.

There is an argument, which Professor Carol Harlow has eloquently espoused, that: third-party interventions are to be discouraged; litigation exists to determine issues between the parties; and democratic institutions, such as parliament, should determine matters relating to the public. The US presents, in this respect, somewhat of a warning. In this country, battles about the right to life, for example, are fought in parliament. Courts, both domestic and European, have fought shy of getting involved in such matters as the law on suicide or abortion – and long may it remain so. Justice intervened neither in the Pretty nor the Purdy cases: the manner of death is for parliament to decide, not judges.

But, the issues in which we have intervened surely show the proper scope for judicial decision: conditions for deportation; the use of special advocates in parole proceedings; destitution of asylum seekers; a series of cases about control orders; a number of issues about the impact of the European Convention on Human Rights in relation to the military in Iraq; the extent of police duties to protect life; and the exercise of prosecutorial decision.

The UK courts remain lions under the throne of parliament. None of these cases overturned UK legislation, even when we were successful in persuading the court. Thus, the ultimate power of an intervenor is more limited than in the US. Our conclusion is clear and unapologetic: democracy benefits from the enhanced scrutiny of the courts that intervenors can encourage.

The Justice report can be downloaded at www.justice.org.uk.

Roger Smith is director of law reform and human rights organisation Justice