How many law lords does it take to decide a case? Normally, the answer is five. But last week and this, nine members of the Appellate Committee of the House of Lords are hearing two important cases. When the new Supreme Court opens its doors in October 2009, seven or nine justices should hear every case.

There are 12 law lords, if you count (and no one has done so for a long time) Lord Saville of Newdigate, who has spent the past ten years out of the office chairing the inquiry into the 1972 Bloody Sunday shootings in Northern Ireland and who is unlikely ever to return to judicial work even when (if ?) he finishes his report. But the remaining 11 law lords never squeeze around the same table to hear appeals. Almost all appeals are heard by five law lords. Exceptionally, nine of them listened to argument last week in a case brought by the President of the republic of Equatorial Guinea against defendants who he alleges conspired in England and elsewhere to overthrow the Government and seize power by means of a coup which, in the event, failed. And nine judges are this week hearing a case brought against the Prime Minister by two mothers whose sons were servicemen killed on duty in Iraq and who contend that there should be an inquiry into whether the invasion was in breach of international law.

In other important cases in recent years, nine law lords decided the human rights implications of delay in criminal proceedings, whether the Hunting Act 2005 is valid law and whether terrorist suspects could be detained lawfully without trial. Enlarged Committees sometimes sat in the past. In 1897 nine law lords heard the trade union case of Allen v Flood, and incidentally also had eight High Court judges sitting with them and assisting in their deliberations. Seven law lords have sat on occasions deciding, for example, the cartel case of Mogul Steamship in 1891, the libel case of Cassell & Co v Broome in 1971-1972 and, more recently, whether evidence obtained by torture abroad could be used in terrorist cases.

There are two main reasons why seven or nine law lords should hear all the appeals. The first is that the Appellate Committee, like Moses when the Children of Israel were travelling towards the Promised Land, decides “the hard cases”. The House of Lords has explained that its role is not to reverse errors in the application of settled law, and so a refusal of leave to appeal does not mean approval of the judgment of the Court of Appeal. Since the law lords are concentrating on difficult points of law, they are more likely to arrive at the right answer for the right reasons, or at least give the best possible answer when the case concerns a dispute between two rights or between two wrongs, if they draw on the collective brain power and distinct legal experience of more of its members.

The second reason why most of the law lords should be on duty for each case is that to perm 5 out of 12 (or 11) inevitably means that there are closely divided cases where the team selection determines the result. As Lord Hoffmann noted in a 1998 judgment, there are “cases in which one feels that a slight change in the composition of the Appellate Committee would have set the law on a different course”. The Hedley Byrne case in 1963 significantly developed the law on liability for negligent misstatements. The appeal was originally argued before a much more conservative panel, but abandoned after one day to permit Lord Radcliffe to preside over the Vassall Tribunal. Unsurprisingly, given the difficulty and controversy of the issues that come on appeal, cases are frequently decided by the casting vote of one of the five judges.

If more law lords were to hear each appeal, the volume of cases would need to be restricted. The right of the Court of Appeal to grant leave to appeal would have to be removed so that the law lords could manage their own caseload, a particular concern as they also sit in the Judicial Committee of the Privy Council. That jurisdiction is much reduced in size as an increasing number of Commonwealth jurisdictions has understandably decided no longer to have a final court of appeal in London. Retired Court of Appeal judges often make up the numbers on the Judicial Committee. But the role of that committee means that the principled solution – that all Supreme Court judges should sit on every case – is not at present feasible. If that principled solution can be adopted, the number of law lords should be reduced to nine to make the Supreme Court panel manageable, a result achievable by not replacing those law lords who retire.

Walter Bagehot rightly argued in The English Constitution in 1867 that a supreme court “ought not to be hidden beneath the robes of a legislative assembly”. On the new Supreme Court, fewer of the judges should remain hidden in their rooms when their colleagues are deciding important cases.

The author is a practising barrister at Blackstone Chambers in the Temple and a Fellow of All Souls College, Oxford

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