THE Lockerbie bomber's lawyer has criticised the Scottish government's silence over allegations that foreign secretary David Miliband "interfered" in his client's appeal by demanding to keep secret a document which could potentially clear him of the terrorist outrage.

Tony Kelly said Holyrood's failure to publicly discuss the UK government's attempts to block disclosure on the grounds of national security was at odds with first minister Alex Salmond's comments two weeks ago on a prisoner transfer agreement signed between the UK and Libya over the return of Libyan nationals held in British jails.

Last week, the Court of Criminal Appeal in Edinburgh heard that Elish Angiolini, the Lord Advocate, was prepared to reveal the document provided by an unnamed foreign power, which has been uncovered by the Scottish Criminal Cases Review Commission's (SCCRC) review of the case, but kept from convicted Abdelbaset Ali Mohmed al-Megrahi's defence team at his trial.

However, Lord Davidson, QC, the Advocate General who is Westminster's legal representative in Scottish matters affecting the UK, urged at the behest of Miliband that an immunity be issued so the document is not made public on the grounds of national security.

Miliband and Lord Davidson's desire for the document to remain secret led to an astonishing courtroom claim by Maggie Scott QC, for Megrahi's defence team, that the UK government was involved in unwarranted "interference" in a Scottish criminal case.

Last night, Kelly said: "This is the first time we've ever had a (UK public immunity) certificate signed during a criminal process in Scotland. Why aren't the Scottish ministers jumping up and down and screaming from the rooftops about UK interference?

"What does the Crown Office have to say? How can they make a comment one week and not the next?"

Kelly added that the highly sensitive document at the centre of the dispute is potentially so important that it "could form the winning point of his appeal".

The document, thought to contain information about the electronic timer used to explode the bomb aboard Pan Am Flight 103, that killed 270 people in December 1988, was one of six factors in the SCCRC decision to refer Megrahi's conviction to the appeal courts last May.

Meanwhile, pressure intensified yesterday for the document to be disclosed.

Jim Swire, whose daughter Flora died in the atrocity, claimed in a letter published in The Herald that Dumfries and Galloway Police, who investigated the attack, already possessed a document from a foreign power which he believes could contain details of the timer.

He said it could show that the one planted on the flight was a Syrian-made fixed-timer, rather than a "multi-adjustable" device manufactured by the now-defunct firm Mebo, as was claimed at Megrahi's trial. If that was the case, Swire added, it could not have originated in Malta, as the prosecution outlined, because it exploded only 38 minutes after the flight took off from Heathrow.

Swire wrote: "Why would a terrorist in Malta use a timer setting that would only clear Heathrow by 38 minutes, when his alleged device was perfectly capable of lasting until the aircraft would be well out over the Atlantic? And in December, just before Christmas, with two changes of aircraft and endless possibilities for delayed flights."

Professor Robert Black, the architect of the Lockerbie trial, said the supposedly secret document was probably too old to be damaging to the UK or the country where it originated.

He said: "If the Lord Advocate thinks this document ought to be handed over to Megrahi's legal team, it's somewhat surprising the Westminster government is saying no. If this document was handed over to the prosecution in the Lockerbie case in 1996, how realistic is it for UK national security in 2008 to be affected?"

Ex-CIA agent, Robert Baer, who worked on the Lockerbie investigation and believes Iran was responsible, said: "There's no reason why it shouldn't be presented to the court, apart from the fact there may be a contract not to reveal co-operation between two security organisations, such as MI6 and the CIA."

A spokesman for the Scottish government said it "does not offer a running commentary, or indeed, any kind of comment on court cases."

A Crown Office spokesman said: "The Crown made it clear that the lord advocate does not consider the UK government is interfering."

The SCCRC was unavailable for comment. The judges in the case will publish a decision on the issue at a later date.