Peter Swindon and John Ashton

Former justice minister Kenny MacAskill has revealed details of a highly classified secret document which casts serious doubt on the safety of the conviction of Lockerbie bomber Abdelbaset al-Megrahi.

However, the Foreign and Commonwealth Office (FCO) has said that the revelation, which will appear in MacAskill's new book about the downing of Pan Am flight 103, 'might' constitute a breach of the Official Secrets Act.

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It is understood the FCO only became aware that top secret details were disclosed in MacAskill’s book when the Sunday Herald contacted the UK government about the revelations. Officials are now believed to be seeking legal advice.

The person who discloses information is guilty of an offence if they do so “without lawful authority knowing, or having reasonable cause to believe, that it is protected against disclosure”.

In his book, ‘The Lockerbie Bombing: The Search for Justice’, which is due to be released on Thursday, MacAskill reveals details of a secret document which implicates the terror group the Popular Front for the Liberation of Palestine-General Command (PFLP-GC) in the Lockerbie bombing carried out on December 21 1988.

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The PFLP-GC were the original suspects in the investigation into the biggest terrorist atrocity ever to have been committed in mainland Britain, which claimed the lives of 270 people, including 11 Lockerbie residents.

However, by 1991 police and prosecutors were entirely focused on Libya and in 2001 Megrahi – who was a former Libyan intelligence officer - was convicted of the bombing and sentenced to life in prison.

He was released by MacAskill in August 2009 on compassionate grounds due to ill health and died of cancer two years and nine months later in Tripoli.

The significance of the document which implicates the PFLP-GC is played down by MacAskill in his book but it does suggest others may have been involved in the bombing.

Read more: Lawyer calls for fresh appeal over Lockerbie bombing

The details of the document are covered by a strict Whitehall gagging order. The document in question was the subject of a legal wrangle during Megrahi’s second appeal against conviction.

The Scottish Criminal Cases Review Commission (SCCRC) referred the case to the High Court on the basis that there may have been a miscarriage of justice.

That conclusion was reached after the SCCRC team that investigated Megrahi's conviction discovered the existence of the document during their four-year probe which concluded in 2007.

Their 800-page report explains that their investigative team were allowed to access the document in Dumfries police station but they were prevented from removing the notes they made on it and the document itself.

The SCCRC was only able to access the document after signing up to a special agreement not to divulge the contents and was told by the Crown that “a conclusion was reached that the documents did not require to be disclosed in terms of the Crown's obligations”.

When Megrahi’s defence team pushed for the recovery of the information the Lord Advocate took the view that it would be appropriate to disclose the document.

However, the Advocate General, representing the UK government, produced a public interest immunity (PII) certificate signed by then Foreign Secretary David Miliband, which blocked the disclosure on the grounds of national security.

A spokeswoman for the FCO confirmed that “the [PII] certificate is still active” and “if the material protected by the certificate were disclosed, it might constitute a breach of the Official Secrets Act.”

She added: “It would be for the publisher of the book to seek their own legal advice about any legal risks they are running.”

A Scottish Government spokeswoman said: “This is a matter for the publisher to advise upon.”

In 2012 the UK Government went to great lengths to prevent our sister paper The Herald revealing details of the document.

It threatened legal action to stop publication and asked the paper to sign up to a court-approved gagging order.

At that time only the Crown, UK Government and SCCRC team knew the contents of the closely guarded document.

The Herald did publish some details which implicated the PFLP-GC, and revealed that the document originated in Jordan.

MacAskill, however, has gone much further, naming key individuals who were party to the contents of the document, and the potential security ramifications of its release into the public domain.

The Sunday Herald has chosen - after consultation with our lawyers - not to publish the full details of the document despite knowing its contents.

Co-founder of campaigning organisation Lockerbie Truth, Dr Jim Swire, 80, whose daughter Flora was killed in the bombing, said the revelations confirmed his suspicions about the potential involvement of the PFLP-GC.

He said: “It’s exactly what the relatives of the victims have thought for many years. I hope that the book is published without interference. It may lead us to find ways of breaking through the refusal to look again at the evidence used to convict Megrahi.

“This sort of thing - pointing to official knowledge of the real perpetrators - could be absolutely crucial.”

A Crown Office spokesman said: “The Crown has had no involvement in the publication of the former Cabinet Secretary’s book and cannot therefore comment on its content ahead of publication.

“The suggestion that the PFLP-GC was responsible for the Lockerbie bombing was fully considered by the trial court following the incrimination of this terrorist group by Megrahi during his trial and does nothing to undermine the Crown's case that Megrahi acted with others in the bombing of flight Pan Am 103.

“All material which met the Crown's disclosure obligations in relation to the PFLP-GC was properly disclosed to the defence before the trial and this was confirmed by the SCCRC's investigation.

“The court concluded that the conception, planning and execution of the plot which led to the bombing was of Libyan origin. The court was, of course, only dealing with evidence, not matters of opinion or conjecture.”

When asked about the possible breach of the PII certificate, Victoria Gilder, Publicity Director at Biteback, the publisher of MacAskill’s book, said: “Sorry, I can’t comment on that because I don’t know anything about it.

“The book is embargoed until next week…you’re not supposed to run anything. It’s embargoed until Monday.”

Last night a Foreign Office spokesman confirmed that the government department has not seen a copy of the book, but added: “We take the protection of material covered by Public Interest Immunity certificates extremely seriously.”

The Sunday Herald repeatedly contacted MacAskill but he did not respond to a request for comment, and his publisher indicated that he is unable to comment until the book embargo is lifted on Monday.