Prosecutors would be prepared to accept the partial disclosure of a secret document to lawyers acting for the Libyan man convicted of the Lockerbie bombing, provided national security was not put at risk, the Court of Criminal Appeal in Edinburgh was told.

Abdul Baset Ali al-Megrahi's lawyers say that the document will help to release him, but the British Government does not want the document to be made public and has lodged a public interest immunity plea to keep it secret.

Ronnie Clancy, QC, for the Crown, told the three judges deciding the case that they would have to look at ways of reducing any possible prejudice to al-Megrahi if they ruled that the document should not be given in full to his defence. He suggested that limited disclosure, by means of a summary of the document, could be made available if it could be handed over without damaging national security interests.

Al-Megrahi was jailed for at least 27 years for the bombing of Pan Am flight 103 in 1988.

He lost an appeal in 2002, but the Scottish Criminal Cases Review Commission referred the case back to the Court of Criminal Appeal last year.

One of the reasons cited was that the Crown had not disclosed to the defence a document originating from an undisclosed foreign government.

Al-Megrahi's lawyers are trying to get their hands on the mystery document in the belief that it could help to clear him. It is thought to contain information about the electronic timer that detonated the bomb.

David Miliband, the Foreign Secretary, has signed a public interest immunity certificate, which states that making the document public could cause “real harm” to national security and international relations.

Judges have said there should be a hearing to consider the public interest immunity issue.

Mr Clancy told the judges yesterday that if they ruled against full disclosure of the document, the Crown's position was that limited disclosure, by means of a summary or redacted version of the document was “the best option if they can be given to the petitioner [Al-Megrahi] without damaging the national security interests which give rise to the plea”.

He told the court: “The principle about involving the defence to the maximum extent looms large here.

“The Lord Advocate is mindful of the difficulty that the petitioner would have in bringing a ground of appeal without even limited disclosure.”

He acknowledged that Lord Davidson QC, the Advocate General, who represents the Government in legal matters in Scotland, may be “less enthusiastic” about the idea.

But he said: “I don't understand the Advocate General to have ruled out limited disclosure at this stage.”

Lord Davidson QC asked the court on Tuesday to hold a hearing behind closed doors to discuss the confidential document.

He said that a “special representative” - a security-vetted advocate - could represent al-Megrahi's interests at that hearing, in place of his usual defence team.

Al-Megrahi was not present in court.