The Advocate General yesterday made a detailed case for a special security-vetted representative to replace the Lockerbie bomber's defence team in a hearing about a confidential document.

Lord Davidson, who represents the UK Government, told three appeal judges that they would be given access to the top secret material.

However, he said releasing the document would cause "real harm" to national security and international relations.

As revealed in The Herald last week, he wants the hearing which will address the issue of "public interest immunity" to be held behind closed doors and with the presence of a special advocate to represent the interests of Abdelbaset Ali Mohmed al Megrahi.

He said the advocate would have to be "security vetted" before being appointed and added that five senior counsel, as well as other junior counsel, had already been vetted.

It would be the first time that such a system has been followed in Scotland and Lord Hamilton, sitting with Lords Kingarth and Eassie, acknowledged it was a "novel procedure".

The document, which is thought to contain information about the electronic timer used to detonate the bomb, was uncovered during an investigation by the Scottish Criminal Cases Review Commission.

The Crown's failure to disclose the document was one of six grounds on which the case was referred back to the courts.

However, Foreign Secretary David Miliband has refused to allow its disclosure and signed a public interest immunity certificate in relation to the document.

Yesterday was the first day of a three-day procedural hearing to decide how the court should deal with the public interest immunity issue.

Lord Davidson told the appeal judges: "What is proposed on behalf of the secretary is that the material be disclosed to the court in a closed hearing."

In a written submission presented to the court, he said that a special representative could represent the interests of Megrahi and also "assist the courts in a fair hearing"

He disputed arguments about the practicality of introducing special advocates for the first time in Scotland and said it could protect the two conflicting interests of the defence and the government.

Megrahi's appeal is due to be heard before five appeal judges and the Advocate General suggested that the public interest immunity hearing would also have to be heard by a court of five judges.

The defence did not give their written submissions yesterday, but Lord Davidson told the court that the defence appeared to contest the use of a special representative.

Megrahi, who was convicted in 2001 of the bombing which killed 270 people in 1988, was not in court..