The defence team of the Libyan man convicted of murdering 270 people when Pan Am Flight 103 exploded over Lockerbie 19 years ago claim to have evidence suggesting that the key Maltese prosecution witness was allegedly offered a $2m reward from the US in return for his testimony.

The UK’s Guardian newspaper broke the story, saying that these new developments have raised fresh doubts about the safety of the case.

Lawyers for Abdelbaset Ali Mohmed al-Megrahi claim to have evidence that detectives investigating the bombing recommended that Tony Gauci, who owned a shop in Gzira near the time of the bombing in 1988, allegedly be given the payment after the case ended.

Mr Gauci’s testimony at the trial was crucial to al-Megrahi’s conviction. He told the trial at Camp Zeist in the Netherlands that al-Megrahi had bought clothes at his shop which the prosecution claimed were packed into the suitcase bomb that exploded over Lockerbie on 21 December, 1988.

The defence team believes Mr Gauci may allegedly have received a larger sum from the US authorities. His role in the case is to be central to al-Megrahi’s appeal against his conviction, which the Scottish Criminal Cases Review Commission said was unsafe.

The defence team is set to press for full disclosure of information about the payments, as well as the release of a potentially vital US document which is thought to cast doubt on official accounts about the timer allegedly used in the bombing, at an appeal hearing next week.

The secret document is believed to dispute prosecution claims that al-Megrahi used a digital timer bought from a Swiss company and then planted the bomb on a flight from Malta to Germany – a disclosure which would fatally undermine his conviction.