LUCY ADAMS and KEVIN SCHOFIELD

The Crown Office was under increasing pressure last night to make a statement on claims that the prosecution in the Lockerbie trial withheld vital information from Abdelbaset Ali Mohmed al Megrahi's defence team.

Politicians said it was vital that the Lord Advocate, Elish Angiolini, explain to parliament why the report on the timer which allegedly detonated the bomb over the town has still not been disclosed.

The Herald revealed earlier this week that the existence of the report was one of the reasons why the Scottish Criminal Cases Review Commission had ruled that Megrahi's appeal against conviction should be heard.

Calls for Ms Angiolini to make a statement came as it emerged that Lord Hamilton, the Lord President and Scotland's most senior judge, will chair the Lockerbie appeal.

Tam Dalyell, the former Father of the House of Commons and a long-standing Lockerbie campaigner, yesterday said the prosecution's failure to pass on its report to the defence team "threatens the good name of Scottish justice internationally". He said: "The Crown has to be challenged on this because fundamental issues seem to be at stake. It looks as if the Crown has simply not fulfilled what any layman like me would suppose was its obligation to the court."

Alex Neil, the SNP MSP, said it was essential that the Crown Office explains its position.

He said: "I believe the Crown Office has a responsibility to answer these allegations. They cannot wait until next year when the appeal may be held. This has to be clarified now by the Lord Advocate."

Bill Aitken, the Scottish Conservatives' justice spokes-man, said that while he doubted Megrahi's innocence, the allegations needed to be investigated.

He said: "Clearly, if these allegations were to be true, they would impact upon the appeal. But of course, the waters are becoming very muddied."

Last month a review on disclosure commissioned by the previous Scottish Executive said prosecutors should be legally bound to provide full information to defence lawyers in advance of a trial.

Lord Coulsfield, who chaired the review, was also one of three judges who presided at the Lockerbie trial in the Netherlands.

He called for legislation requiring the prosecution to have regard to "the over-riding requirement of a fair trial".

Pauline McNeill, Labour's Shadow Justice Secretary, agreed that the system must be clarified. "I'm concerned about the number of cases where disclosure is becoming an issue, which suggests work needs to be done. Whether it's legislation that's required, or a clearer framework, we need to give the Crown more guidance," she said.

A spokesman for the Crown Office said it would be "inappropriate" to comment on the Lockerbie case while Megrahi's appeal is still to be heard.