The defence team for the Libyan jailed for the Lockerbie bombing yesterday suffered a set-back in its attempts to get access to a top-secret document.

The document, which originated in an unknown foreign country, is thought to contain vital information about the timer which detonated the bomb that killed 270 people in 1988.

At the previous hearing, the UK Government said the document could not be disclosed for reasons of national security, leading the defence team to accuse it of "interference" in the appeal.

Margaret Scott QC, senior counsel for Abdelbaset Ali Mohmed al Megrahi, the Libyan currently serving 27 years in Greenock prison for the bombing, objected to the Advocate General for Scotland - the law officer who represents the UK Government in Scottish affairs - playing a part in the debate.

She accused the government of meddling - an allegation hotly disputed by Lord Davidson, the Advocate General, and by Elish Angiolini QC, the Lord Advocate and head of prosecutions in Scotland.

However, yesterday the appeal judges ruled against her. Their decision opens the way for several days of future debate about whether letting lawyers see the document would have any security implications.

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

Earlier this year, the Court of Criminal Appeal in Edinburgh was told that Ms Angiolini would be prepared to disclose the document but that has also been disputed.

The document itself 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 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.

Foreign Secretary David Miliband claims the document should remain confidential.

Now Lord Davidson will be allowed to put the case for "public interest immunity", on his behalf, at a future hearing - for which no date has yet been set. The hearing of Megrahi's actual appeal is still months away.

Megrahi, who was jailed in 2001, was not in court yesterday - but the appeal judges have been told he would like to attend future appeal hearings.