SCOTLAND'S judges have launched an unprecedented attack on the constitutional position of the country's top prosecutor, suggesting she should be stripped of most of her powers.

In a collective statement in a submission to the Calman Commission on the future of devolution, the judges say there is a fundamental problem with the Lord Advocate - currently Elish Angiolini - being both a member of the government and the independent head of the prosecution service.

They say the Lord Advocate's dual role has led to a "substantial number" of challenges from prisoners on human rights grounds, choking the court system and delaying trials.

The judges suggest stripping the Lord Advocate of her prosecution role, and creating a separate Director of Public Prosecutions instead, as in England and Wales.

They admit this would "rob the Lord Advocate of most of her functions", and leave her a mere legal adviser to the government.

The Lord Advocate has been the most senior public prosecutor in Scotland since 1587.

In recent years, opposition MSPs have also called for the Lord Advocate's roles to be separated.

Shortly after coming into power last year, Alex Salmond acknowledged the disquiet by saying Angiolini would no longer attend cabinet meetings in order to distance her from political discussions.

But she and her fellow law officer, the Solicitor General, remain members of the government - a position that can only be changed by amending the Scotland Act which underpins devolution.

Bill Aitken, Tory justice spokesman, said: "There have been concerns for some time that the present constitutional position was causing difficulties.

"It's time there was a review. The problem is how we remedy this. As the judges have already anticipated, it is not so simple."

Robert Brown, for the Liberal Democrats, also demanded a review: "The position is increasingly anomalous in an age when people are concerned about the separation of powers. We need an analytical review, but we need to get it right. We should not rush it."

Under the Scotland Act, the Lord Advocate is a member of the Scottish government and must do nothing which is incompatible with the European Convention on Human Rights (ECHR).

However, the Lord Advocate is also the head of the Crown Office. The two roles frequently clash, generating what are known as "devolution issues".

When prosecution decisions appear in conflict with the duty towards ECHR, these devolution issues can be appealed in a higher Scottish court or ultimately to the Judicial Committee of the Privy Council in London.

For instance, in numerous appeals since 1999, those convicted of crimes have argued they received an unfair trial - in breach of Article 6 of ECHR - because the Lord Advocate, as head of the prosecution service, failed to disclose material evidence.

On Friday, Nat Fraser, who was convicted in 2003 of killing his wife, asked for leave to appeal to the Privy Council over a similar devolution issue.

In their report, the judges say the Lord Advocate's dual roles have generated scores of such challenges, gumming up the justice system. The opportunity "to challenge... virtually any act of a prosecutor has led to a plethora of disputed issues, with consequential delays to the holding of trials and to the hearing and completion of appeals against conviction."

The involvement of the Privy Council "has arguably created, or at least contributed to, the delay in the handling of criminal business," they add.

It is a long-running complaint among judges that their decisions can be overturned by the Privy Council, which decides on whether a person received a fair trial.

In Scotland, the test is higher: whether a person suffered a miscarriage of justice.

Solicitor Advocate John Scott of Capital Defence Lawyers, an authority on human rights law, said the judges' proposals posed a risk.

"The position of the Lord Advocate is overly-complicated. Unfortunately, in dealing with that the judges might also end up removing essential safeguards, like appeals to the Privy Council. People's rights are better looked after there than they are in Scotland."

The judiciary offer three possible solutions to the problem, but do not come down in favour of any particular one.

They write: "Her responsibilities as the public prosecutor could be transferred to an independent Director of Public Prosecutions' in Scotland, who would be responsible for the prosecution system, but who would not be a member of the Scottish Executive (sic).

"Such a change would rob the Lord Advocate of most of her functions, but would leave the Scottish Executive with a Lord Advocate who was a general legal adviser to the Executive."

They also suggest Westminster could amend the Scotland Act to explicitly exempt the Lord Advocate's actions as a prosecutor from compliance with ECHR.

A third possibility would be changing the law on criminal appeals, although they warn "such a radical nature would be likely to generate considerable controversy".

A government spokesman said ministers would study the judiciary report closely.