Crucial information about a fragment of electrical circuit board that was alleged to have come from the bomb which destroyed a passenger aircraft over the skies of Lockerbie, Scotland, killing 270 people in 1988, was given to police in the run-up to Megrahi's trial in 2000 but never disclosed, it is claimed.
The allegations are made in the book Megrahi: You Are My Jury, by John Ashton. The book has been condemned by David Cameron, who called it "a disgrace" to the families of the murdered. It claims that a key fragment of circuit board, found at the Lockerbie crash site and said by the prosecution to be from a timer which detonated the bomb, could not have been one of a batch that was sold to Libya by the manufacturers.
The fragment was a vital link in the prosecution argument that the bomb was placed in the aircraft by Megrahi. Last night experts who have closely followed the case said the claim, if true, meant the case against Megrahi is now "blown out of the water".
During Megrahi's trial it was accepted the fragment from the timer came from the Swiss company Mebo. The company admitted selling 20 such timers to the Libyans, but new evidence points to the Lockerbie fragment not being one of them. The one at Lockerbie was coated in tin, whereas those sold to Libya were coated with a tin and lead alloy, Mr Ashton says. A sworn affidavit from the production manager said the company only ever used alloy, rather than pure tin.
Megrahi's trial heard evidence from two prosecution witnesses that the lack of lead on the coating could be explained by it having been burned off in the heat of the explosion. Neither witness was an electronics expert.
However, the book reveals that Megrahi's solicitor, Tony Kelly, commissioned two scientists, Dr Chris McArdle, a former adviser to the Government, and Dr Jess Cawley, a consultant to the engineering industry, to test the suggestion. Both concluded this could not have happened.
The book also claims that notes by a prosecution forensics expert, Alan Feraday, during his original examination of the circuit board fragment in 1991, reveal he was aware of a difference in the make-up of the circuit board. However, his notes, which were given to police on 8 November 1999, were not disclosed to Megrahi's defence team until 2009.
"Had these documents been disclosed to the defence team, they would have provided the basis for a vigorous cross-examination of Feraday but, in the event, his claim that the fragment was 'similar in all respects' to the control samples went unchallenged," said Mr Ashton. "I don't believe the police would have withheld the documents from the Crown, which raises the second question: why was it not disclosed to the defence?
"Whether it was deliberate or not, I don't know. But it was appalling, and someone should be held to account for it. They did not meet their duty of disclosure. That is a huge scandal."
The Independent on Sunday sent the relevant pages of the book to Mr Feraday but received no response.
Defence lawyer Gareth Peirce said yesterday: "What the research makes unarguable is that any claimed investigation to date has been determinedly false and has robbed them of a truthful and transparent account."
Peter Biddulph, a researcher for Jim Swire, who lost his daughter in the tragedy, said the allegations would further victims relatives' push for a new inquiry. He said: "[These allegations] show the case against Megrahi is totally blown out of the water."
A Crown Office spokesperson said: "In respect of the timer fragment, the defence experts were satisfied it had suffered damage consistent with it having been closely associated with an explosion and that it had come from an MST-13 timer."