Prosecutors will next week attempt to throw an unprecedented veil of secrecy over the appeal of the Lockerbie bomber.

The Crown Office will ask judges to bypass the defence team of Abdelbaset Ali Mohmed al Megrahi and appoint special security-vetted advocates to represent him in a court hearing to decide whether a previously confidential document should be made public.

If the bid for a closed-door session is successful, it would be the first time in Scotland that such a step has been taken in a criminal case.

However, the tactic will fuel suspicions that the Crown is going to unusual lengths to preserve the UK's current diplomatic relations with other nations.

The paperwork, which originated in an unknown foreign country, is thought to contain vital information about the electronic timer which detonated the bomb that killed 270 people in the skies over Lockerbie.

It is not known if political pressure has been exercised directly on the Crown, but there have been previous instances in the Megrahi case where Britain's changed attitudes to foreign states since 1988 have played a key role in the legal process.

Foreign Secretary David Miliband has already said the document should remain confidential.

It 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 the document could constitute a miscarriage of justice. Although the Crown allowed the commission to see the material, it has refused to disclose it to Megrahi's defence team.

The Crown's latest move is expected to anger further his lawyers, who believe the failure to disclose the document calls into question the ultimate right to a fair appeal.

The request will be made on Tuesday at the Court of Criminal Appeal when the decision on whether to grant the defence access to the document is to be debated.

The Crown is expected to ask for the hearing to be held behind closed doors in the absence of the defence, who would be represented by special advocates. Public Interest Immunity hearings of this kind in criminal cases have previously been held only south of the border, where there is a statutory system in place, and a list of special advocates.

Megrahi's defence team has made it clear that it needs to see the document in order to proceed with the appeal, and has accused the UK Government of "interference" in the appeal.

If the prosecution denies access to the paper, Megrahi's lawyers are expected to argue that the conviction should be quashed because, without it, their client's right to a fair trial would be breached.

One legal expert said: "This is entirely unprecedented in Scotland."

A spokesman for the Crown Office said the court hearing is to be from from May 27 to 29 in Edinburgh. "It is not possible to provide further comment," he said.