The wrongful convictions of hundreds of subpostmasters and subpostmistresses for financial crimes has rightly given rise to great public indignation and demands that such convictions should be quashed as soon as possible. No one suggests that a wrongful conviction should not be quashed: the problem which arises is how this should be done consistent with the rule of law.

A central aspect of the rule of law is the separation of powers, which means that the executive, the legislature and the judiciary are clearly divided and that none of the three branches of these institutions of the state can exercise the power of the other. The convictions of the subpostmasters and subpostmistresses were acts of the courts, or in other words of the judiciary, and if these convictions were wrongful they can be quashed only by the judiciary in the form of a court of criminal appeal.

It does not lie within the power of the legislature, whether that be the UK Parliament or the Scottish parliament, to pass an enactment quashing a conviction in a specific case or specific cases. To do so would amount to a blatant usurpation of the judicial power of the courts and a contravention of the doctrine of the separation of powers. The quashing of a criminal conviction is the exclusive function of the judiciary in the form of a court of criminal appeal.

If there is concern about the length of time it will take for the court to deal with a particular appeal coupled with the length of time which has passed since the wrongful conviction, as is the case with the wrongful convictions of subpostmasters and subpostmistresses, then the court can always be asked to expedite or accelerate the hearing of the appeal, especially if the prosecution accepts that the conviction should be quashed.

In Scotland the lord advocate has stated (contrary to the views expressed by the first minister) that due process must be followed in the quashing of wrongful convictions. She was absolutely correct to do so. It is proper that each conviction, and the evidence relied upon for it, should be considered separately. In the case of an appeal against a summary conviction in Scotland where the prosecutor is not prepared to maintain the judgment appealed against he may, by a relevant minute, consent to the conviction being set aside (section 188(1)(a) of the Criminal Procedure (Scotland) Act 1995).

Moreover, where no such appeal has been taken but the prosecutor is, at any time, not prepared to maintain the judgment on which a conviction is founded he may, by relevant minute, apply for the conviction to be set aside (section 188(1) (b)). These powers have been used in the past in cases where numerous drivers were wrongly convicted of speeding on the basis of a misunderstanding that a certain speed limit was in force when in fact it was not. There are no equivalent provisions in the case of a solemn conviction where it is for an accused who maintains that he or she has been wrongfully convicted to bring an appeal against conviction (out of time with the permission of the court if required).

The prosecution will then consider the evidence relied upon for conviction and if it is satisfied that a conviction should not have been returned on the basis of that evidence it will so state to the appeal court with a suitable explanation and invite the court to quash the conviction. If the prosecution is satisfied that the conviction was properly secured then the case will proceed to an opposed appeal hearing in the normal way and the court will decide whether or not the conviction should be quashed.

It is not within the competence of the legislature at either Westminster or Holyrood to override this established legal procedure and to take upon itself the task of quashing convictions which it considers to have been unlawful. Whether a conviction should be quashed is a matter for judicial determination, not legislative action. A political rush to right many miscarriages of justice must not result in the rule of law and the separation of powers being cast aside. It is in the interests of all of us, including those who were wrongly convicted, that the rule of law and the separation of powers should prevail.