A top secret document vital to unearthing the truth about the Lockerbie bombing was obtained by the Crown Office but never shown to the defence team, The Herald can reveal today.

Prosecutors have refused to make public the report, which is classified as confidential under national security guidelines and could apparently fatally undermine the case against Abdelbaset Ali Mohmed al Megrahi.

The Scottish Criminal Cases Review Commission (SCCRC) team that investigated Megrahi's conviction is understood to have discovered the existence of a document about the MST13 timer which allegedly detonated the bomb over Lockerbie in 1988, killing 270 people.

Using its enhanced powers, the commission compelled the Crown to show it the highly confidential document and decided the contents - still unknown to the defence - were sufficiently disturbing for a court to have believed the conviction could have been a miscarriage of justice.

Proving the MST13 timer found at the site was purchased by the Libyans was pivotal to the conviction at Camp Zeist in the Netherlands. It is thought the document originally came from the CIA and questions the validity of claims the timer was bought by the Libyans.

It is also thought to dispute whether it was the same timer used to detonate the bomb and suggests other countries and terrorist networks would have had access to such a device.

Members of the Crown Office are understood to have signed an agreement with the US security agencies at the time to say that, if they viewed certain confidential documents, they would not disclose the details. The defence team is expected to cite the secret report and the Crown's continued refusal to make it available, as grounds for a special hearing later this month.

A source close to the case said: "The SCCRC has uncovered there is a document which was in the possession of the Crown and was not disclosed to the defence, which concerns the supply of MST13 timers. Moreover, the commission has determined the decision to keep the document from the defence may have constituted a miscarriage of justice.

"The commission was unable to obtain authority for its disclosure. Without access to this document, the defence are disabled from putting before the court full and comprehensive grounds of appeal as to why the conviction should be quashed."

Megrahi, serving 27 years in HMP Greenock, and his lawyers are expected to lodge a court order to compel the Crown to hand over the document later this month. Grounds for his appeal are expected to be put before the courts later this year.

Professor Robert Black QC, one of the architects of the Lockerbie trial at Camp Zeist, said: "If a foreign intelligence agency says they would be prepared to give the Crown access only if they promise to keep the information secret, then it is the responsibility of the Crown to say we cannot do that. They have an ethical responsibility not to sign such agreements.

"This current refusal tends to indicate that the Crown has not changed its fundamental stance that says they will decide what the public interest is, and what information should or should not be disclosed. That is fundamentally wrong."

A spokesman for the Crown Office said: "It would be wholly inappropriate for us to comment while the case is before the appeal court." Megrahi's solicitor, Tony Kelly, refused to comment.