The UK Government has been accused of "interference" in the appeal of the man convicted of the Lockerbie bombing.

The charge was made yesterday as it was revealed for the first time that Scotland's top prosecutor would be prepared to release a top secret document which could overturn the case, but that UK ministers are blocking the move.

The Court of Criminal Appeal in Edinburgh was told that Elish Angiolini, the Lord Advocate, would be prepared to disclose the document which was uncovered during the three-year investigation of the Scottish Criminal Cases Review Commission which resulted in the case being referred back to the courts for a new appeal last summer.

The commission concluded the failure during the original trial to disclose this document, which comes from an un-named foreign country and is thought to contain information about the electronic timer used to detonate the bomb, could constitute a miscarriage of justice.

Although the Crown allowed the commission to see the material they have refused to disclose it to the defence.

Abdelbaset Ali Mohmed al Megrahi is currently serving 27 years in Greenock prison for the 1988 bombing which killed 270 people.

The Libyan's defence team say they need to see the document in order for Megrahi to have a fair appeal.

Maggie Scott, QC, leading Megrahi's defence team, said yesterday that according to the response from the Crown Office, "the Lord Advocate has decided that she should disclose this document for the purposes of the appeal".

She argued the lord advocate ultimately has the jurisdiction in deciding whether to disclose a document in a Scottish criminal appeal.

However, Lord Davidson, QC, the Advocate General, who represents the Westminster government in legal matters north of the border, had said no.

Ms Scott said: "No public interest objection has been taken or raised by the lord advocate. In these circumstances, the only reasonable inference is that ... the lord advocate on reflection does not consider there is a well-founded public interest objection to the disclosure of the document sought."

Ms Scott added: "When one understands this position, it becomes obvious ... the advocate general's intervention is preventing that disclosure.

"But for his intervention the document would be disclosed and when one understands that one sees the intervention by the Advocate General is interference by the UK Government in the pursuit of, the conduct of, a criminal prosecution by the lord advocate."

The advocate general is trying to invoke "public interest immunity" to keep the document secret but Ms Scott said it was incompetent for him to do so.

However, Lord Davidson claimed national security was at stake. He accused Ms Scott of "flawed logic" and said her claim the role of the Advocate General was to interfere in matters which should be left to the Scottish authorities was "wholly erroneous".

He told the court: "This is not a minor squabble. It is one of the most important issues that can ever come before a court. It is a question of national security."

The court heard that David Miliband, the Foreign Secretary, is behind moves for a public interest ruling.

Advocate depute Ronald Clancy, QC, for the Crown, also attacked Ms Scott's arguments, saying the lord advocate had not given up any of her independence.

Scotland's top judge, Lord Hamilton - sitting with Lords Kingarth and Eassie - will issue a decision at a later date about whether Lord Davidson has a right to ask them to keep the disputed document secret.

Megrahi was not in court for yesterday's hearing, but the defence said he would like to attend future appeal hearings, raising questions about where the hearings might be held.