The former Conservative minister described Tony Gauci, the Maltese shopkeeper whose testimony was central in securing a conviction against Abdelbasset Ali Mohmed al-Megrahi, as “not quite the full shilling” and “an apple short of a picnic”.

Fraser, who as Scotland’s senior law officer was responsible for indicting Megrahi, says he is now not entirely happy with the evidence against Megrahi during his trial in 2001 and in his subsequent appeal.

While making clear that this does not mean that he believes Megrahi was innocent of the 1988 atrocity, in which 270 people were killed, Fraser said he should be free to leave Scotland to serve the remainder of his sentence in Libya.

His intervention is the most significant yet in a series of developments that have cast doubt on the safety of the conviction against Megrahi.

Pan Am flight 103 blew up over Lockerbie on December 21, 1988 after an explosion in the cargo hold. Megrahi was sentenced to 27 years following a trial presided over by three Scottish judges in the Netherlands. A condition of his sentence was that he served the full term in Scotland. His co- accused, Al-Amin Khalifa Fhimah, was cleared.

Lawyers acting for the former intelligence officer and head of security for Libyan Arab Airlines have since claimed to have uncovered anomalies suggesting that vital evidence presented at the trial came from tests conducted months after the terror attack. The new evidence is due to be presented in an appeal to the Scottish Criminal Cases Review Commission next year.

Earlier this month it was reported that officials from Britain, America and Libya had met to discuss moving Megrahi back to Libya on the condition that the appeal is dropped.

A key plank in the case against Megrahi was provided by Gauci who claimed that he sold Megrahi clothes that were believed to have been wrapped around the bomb. Fraser said that he believes Gauci was a “weak point” in the case and has expressed concern that he was a “simple” man who might have been “easily led”.

“Gauci was not quite the full shilling. I think even his family would say (that he) was an apple short of a picnic. He was quite a tricky guy, I don’t think he was deliberately lying but if you asked him the same question three times he would just get irritated and refuse to answer,” he said.

“You do have to worry, he’s a slightly simple chap, are you putting words in his mouth even if you don’t intend to?” Fraser said he has been invited to Tripoli to meet Colonel Gadaffi after the Libyan leader learnt of his views but, so far, he has declined.

“I wasn’t particularly impressed with his defence. Their techniques of muddle and confusion can work for a jury but it doesn’t work for three judges,” he said.

Fraser said he believes that Megrahi should now be free to return to his native Libya to see out the remainder of his sentence.

“The transfer of prisoners is quite common but it’s important that you follow the rules of the transferring country. If he is transferred to any country I would expect him to serve out the sentence that the Scottish court imposed,” Fraser said.

William Taylor QC, the man who led Megrahi’s defence, said Fraser should never have presented Gauci as a crown witness: “A man who has a public office, who is prosecuting in the criminal courts in Scotland, has got a duty to put forward evidence based upon people he considers to be reliable.

“He was prepared to advance Gauci as a witness and, if he had these misgivings about him, they should have surfaced at the time.

“The fact that he is now coming out many years later after my former client has been in prison for nearly 4½ years is nothing short of disgraceful.”

Jim Swire, spokesman for the families of victims and who lost his daughter Flora in the atrocity, said: “Lord Fraser had detailed knowledge of events and I think we have to take seriously anything he says now that is relevant to those who gave evidence at Zeist. It is significant that a man who has been as close as he has to the investigation should be making comments like this.”

Gauci said; “I am not interested in what this man said. What matters to me is what the court said and that’s it. That’s all I have to say.”

All the members of Megrahi’s defence team were approached but have declined to comment.

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