Over the course of 821 pages, the Scottish Criminal Cases Review Commission's statement on the case of Abdelbaset Ali Mohmed al Megrahi exposes the Scottish criminal justice system – and in particular the Crown Office – to one of the greatest challenges it has ever faced.

During a four-year investigation the commission uncovered numerous items of evidence that cast doubt on Megrahi's guilt, which the Crown Office had failed to disclose to the Libyan's lawyers during his trial in 2000-1. The SCCRC said the non-disclosure provided four of six separate grounds for referring the conviction back to the appeal court.

Most of the debate around the case has focused on Justice Secretary Kenny MacAskill's decision to grant Megrahi's release. Publication by The Herald of extracts from the report will turn the spotlight on the Crown Office.

Relatives of the UK Lockerbie victims want a public inquiry into Megrahi's conviction. These revelations could make the case unarguable.

The case against Megrahi

The Crown case was that on the morning of December 21, 1988, he smuggled an unaccompanied suitcase containing a bomb on to a flight from Malta to Frankfurt. There the case was transferred to a Pan Am feeder flight to Heathrow, where it was loaded on to Pan Am flight 103. The suitcase contained clothes, which he was alleged to have bought from a shop in Malta called Mary's House, when he visited the island on December 7, 1988.

The shopkeeper, Tony Gauci, recalled selling a collection of similar clothes to a Libyan man a few weeks before the bombing.

More than two years later he picked Megrahi from a photo-lineup, telling police he resembled the purchaser. In 1999, after Megrahi had surrendered for trial, Gauci again picked him out, from a live identity parade.

The six grounds of referral:

1. Unreasonable verdict

The trial judges were satisfied the purchase of the clothing in the suitcase containing the bomb was on December 7, 1988, Megrahi's only window of opportunity.

Gauci was clear it was raining as the man was leaving the shop, yet the court heard from Malta's chief meteorologist that no rainfall was recorded that day.

In the SCCRC's view this rendered the judgment unreasonable: "The Commission has reached the view that the trial court's verdict is at least arguably one which no reasonable court, properly directed, could have returned.

"In particular the Commission does not consider there to be any reasonable basis for the trial court's conclusion that the purchase took place on December 7, 1988, and therefore for the inference it drew that the applicant was the purchaser of the items from Mary's House."

2. Undisclosed evidence about the Gauci identification

Days after Megrahi surrendered for trial, Gauci picked him out at an identity parade. One of Gauci's neighbours had earlier shown him a magazine article, which contained a photograph of Megrahi under the heading: Who planted the bomb?

The SCCRC discovered a statement by Maltese officer Sergeant Mario Busuttil, which showed Gauci had only handed it to the police days before the ID parade. It also uncovered a police report which revealed Gauci had seen other articles with Megrahi's photograph a few weeks before the parade.

The report states: "Crown Office confirmed to the Commission that this report was not disclosed to the defence ... Crown Office has no record of the document in its files but ... Jim Brisbane was confident that as a member of the joint police and prosecution investigation team in Malta in early 1999 he would have been made aware of the information it contained... the Commission is of the view that Sergeant Busuttil's police statement and the report of March 20, 1999, should have been disclosed to the defence. Both items were likely to have been of material assistance in the proper preparation or presentation of the applicant's defence and to have been of real importance in undermining of the Crown case."

3. Undisclosed evidence concerning the date of the clothes purchase

At trial Gauci said the clothes were bought about a fortnight before Christmas, around the time Megrahi was on the island. When interviewed by the defence he said he thought the date might be November 29. The SCCRC discovered an undisclosed Crown precognition statement in which he repeated the November 29 claim, but added that he believed this might be the date.

The report states: "By letter dated August 24, 2006, Crown Office confirmed to the Commission that the contents of Mr Gauci's Crown precognition were not disclosed to the defence. According to the letter this was consistent with the Crown's practice at the time - In the Commission's view by withholding this information the Crown deprived the defence of the opportunity to take such steps as it might have deemed necessary."

4. Undisclosed evidence about Gauci's interest in rewards

The SCCRC discovered three police documents that indicated that, before first picking out Megrahi from a photo line-up in 1991, Gauci was aware a substantial reward was on offer from the US Government and had "expressed an interest in receiving money".

A June 1999 document described him as being "somewhat frustrated that he will not be compensated in any financial way for his contribution to the case" and said his influential brother Paul "has a clear desire to gain financial benefit".

The report states: "Crown Office confirmed to the Commission that [Detective Chief Inspector Harry] Bell's memorandum of February 21, 1991, was not disclosed to the defence. According to the letter, Crown Office has no record of this document in its files and no one there who dealt with this part of the case has any recollection of having seen it before. Crown Office confirmed that the report by Strathclyde Police dated June 10, 1999, was also not disclosed to the defence ... in a further letter, dated April 27, 2007, Crown Office explained that although a copy of the report could not be found in its files, given its nature the possibility could not be excluded that a copy was made available to them ...

"Inquiries with D&G [Dumfries and Galloway Police] have established that, some time after the conclusion of the applicant's appeal against conviction, Anthony and Paul Gauci were each paid sums of money under the Rewards for Justice programme administered by the US Department of State- the Commission is of the view that Mr Bell's memoranda and the passages from Strathclyde Police report ... ought to have been disclosed to the defence. Taken together, all three items were likely to have been of material assistance to the proper preparation or presentation of the applicant's defence and were likely to have been of real importance in undermining the Crown case. Such a challenge may well have been justified, and in the Commission's view was capable of affecting the course of the evidence and the eventual outcome of the trial."

5. Undisclosed secret intelligence documents

In 2006 the Crown informed the SCCRC of two classified documents in its possession. Commission investigators were permitted to view the items at Dumfries police station and take notes, but the notes were left with police. The SCCRC said non-disclosure of one of the documents indicated a miscarriage of justice may have occurred. It did not have permission to reveal the contents of the documents or take them from the police station. Megrahi and his legal team have still not seen this document or been told what it contains.

The report states: "By letter dated April 27, 2007, Crown Office confirmed that neither of the protectively marked documents was disclosed to the defence. According to Crown Office's letter, 'the conclusion was reached the documents did not require to be disclosed in terms of the Crown's obligations'."

6. New evidence concerning the date of clothes purchase

In his police statements, Tony Gauci was clear the purchase was made around the time the municipal Christmas lights had been put up. Evidence emerged since the trial that calls into question Gauci's account. The key document was the diary of Malta tourism minister Michael Refalo which showed he switched on the lights on December 6.

The report states: "The Commission considers Dr Refalo's account is capable of being considered as credible and reliable by a reasonable court, and is likely to have had a material part to play in the determination by such a court of a critical issue at trial, namely the date on which the items were purchased from Mary's House."

l John Ashton is the author of Megrahi You Are My Jury, Megrahi's official biography.