Tony Gauci, a Maltese shopkeeper, became the Crown’s key witness in the conviction of Abdelbaset Ali Mohmed al Megrahi, and was the one man who linked the suspect to clothes found in the suitcase that harboured the bomb.

But new allegations published yesterday, which would have been tested in court if the appeal that began in April had gone ahead, have ­undermined both his ­credibility and reliability.

Papers on Megrahi’s website reveal that Gauci and his brother Paul were interested in financial reward from the start of the case, and that between them they received at least $3m (£1.88m) at the end of the trial.

Previously-secret police reports dating back to 1999 indicate “the frustration of Tony Gauci that he will not be compensated” and that “in respect of Paul Gauci, it is apparent from speaking to him for any length of time that he has a clear desire to gain financial benefit from the position he and his brother are in relative to the case.

“As a consequence he exaggerates his own importance as a witness and clearly inflates the fears he and his brother have.

“He is anxious to establish what advantage he can gain from the Scottish police.

“Although demanding, Paul Gauci remains an asset to the case but will continue to explore any means he can to identify where financial advantage can be gained.”

Offering witnesses financial remuneration is anathema to the Scottish system, and yet this information, uncovered by the investigation of the Scottish Criminal Cases Review Commission, was never disclosed to the defence.

Megrahi’s website states: “It is a matter of common sense, and it has long been recognised in Scots law, that the existence of a financial interest and/or the offer of rewards to a witness is of considerable importance in relation to the credibility of that witness.

“Depending upon the nature and degree of any such interest or reward, the law may exclude the evidence of the witness, or leave the effect of same on the witness to be weighed by the jury.”

The now published papers suggest the police knew this. One quotation is telling: “When the inevitable reflections and media examinations take place in future years … nor is it anticipated would they ever seek to highlight any remuneration received”.

Megrahi’s website summary also states: “The documents also indicate that Tony Gauci had been visited by the Scottish police on more than 50 occasions – many, perhaps even the majority, of which were unrecorded.

“This information shows that the witness has significantly changed his position over time regarding the items sold.

“In addition there is a clear inference from the timing and context of these inconsistent statements that the witness has been influenced in his recollection by the police inquiries – either by being shown articles such as control samples or fragments or by discussion.”

Expert reports published for the first time on the website also question the validity of Mr Gauci’s identification of Megrahi.

A report from Tim ­Valentine, professor of psychology at the University of London, claims that the first identification by Mr Gauci – in which he did not directly identify Megrahi – would have been the most accurate.

It also says that the circumstances before the line-up generated “a serious risk of mistaken identity”.

Steven Clark, professor of psychology at the University of California, and an expert in eyewitness identification, concluded that it would be “extremely unusual” for Mr Gauci to have made a correct identification, because of the passage of time since the incident (nine months before the first questioning) and all the other influences including the numerous newspaper and magazine reports he saw.