IT has been hidden, blocked and kept secret by the UK Government for more than 20 years, but The Herald can reveal for the first time the contents of the top-secret Lockerbie document that the UK tried to prevent us from publishing.

The highly classified document, which has never even been aired in public or shared with the courts, originally came from Jordan and indicates that a Palestinian terrorist group was involved in the bombing that killed 270 people – something the UK Government has vehemently denied.

The UK Government has gone to considerable lengths to prevent details of the document – which casts further doubt on the safety of the conviction of Abdelbaset Ali Mohmed al Megrahi – being published by The Herald.

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

Our decision to publish details of the document, which was obtained by the Crown Office but never shown to the defence team, will prove highly embarrassing to the Crown, the Foreign and Commonwealth Office and the Office of the Advocate General, whose lawyers have worked tirelessly to prevent it ever being even discussed in public.

The document incriminates the Popular Front for the Liberation of Palestine – General Command (PFLP-GC) in the Lockerbie bombing.

The PFLP-GC were the original suspects in the investigation into the biggest terrorist atrocity ever to have been committed in mainland Britain. However, by 1991 police and prosecutors were entirely focused on Libya. Since then politicians and the investigating authorities have denied the possibility of their involvement, instead blaming the whole atrocity on Libya.

Repeated, high-level attempts to block the report indicate it is vital to unearthing the truth about the Lockerbie bombing. The UK Government arranged for the document to be covered by Public Interest Immunity on national security grounds. This prevented it from being shared with the defence but does not prevent publication by a newspaper.

A source said: "The document itself is historical and regimes have changed so it is hard to believe it presents any risk at all to national security. It originates from Jordan and incriminates the Palestinian terror group the PFLP-GC. The contents are very important but what makes them so much more significant is the lengths the UK Government and others have gone to in order to prevent anyone from seeing the document.

"This is the most remarkable piece of evidence. It does not rule out the Libyans but it does indicate that others were involved.

"It also shows the lengths the UK Government was prepared to go to in order to ensure that any evidence undermining their case against Libya would never see the light of day."

It is thought the document could fatally undermine the case against Abdelbaset Ali Mohmed al Megrahi. He died of cancer last month without knowing the contents of this report.

The Scottish Criminal Cases Review Commission (SCCRC) team that investigated Megrahi's conviction discovered the existence of the document during their four-year investigation 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 prohibited from removing the notes they made on it and the document itself.

The commission 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".

The SCCRC then ruled that the contents were sufficiently disturbing for a court to have believed the conviction could have been a miscarriage of justice. The failure to disclose the document was one of the six grounds on which the case was referred back for a fresh appeal in 2007.

To date, only the Crown, UK Government and SCCRC team know the contents of this closely guarded document.

Megrahi's legal team pushed for disclosure of the document once it was revealed by the SCCRC. The Scottish courts were in the process of appointing special advocates and a special judge who would decide in secret whether the contents of the document could be disclosed when Megrahi dropped an appeal in 2009 in order to speed his return to Libya. He was released on compassionate grounds in August 2009 because he was suffering from terminal prostate cancer.

This new evidence and the fact it points to the guilt of non-Libyans will also prove embarrassing for the prosecution who failed to share the document with the trial court and who have subsequently argued that the investigation should focus on Libya alone.

A spokesman for the UK's Foreign and Commonwealth Office said: "The UK Government provided all relevant information on the Lockerbie bombing to the Scottish authorities, who considered it as part of the investigation which led to Megrahi's conviction. Any suggestion of 'hiding' documents is simply incorrect.

"The Government entered into a dialogue with The Herald in line with its long-established practice, supported by successive governments, to seek to prevent publication of any material that could cause significant harm to the UK's international relations and national security. We have consistently made clear that we sought to do this through dialogue rather than legal action".

Chapter 25 of the Scottish Criminal Cases Review Commission's Statement of Reasons refers to material which the Crown had proactively disclosed to the Commission during its review of Megrahi's conviction. The Crown claim they wanted to provide this information to Megrahi's legal team during the second appeal and made this clear to the court, but this could not be done because of the UK Government's Public Interest Immunity Certificate.

A spokesman for the Crown Office said: "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."