Three months ago, the long-anticipated and most high-profile decision of the Scottish Criminal Cases Review Commission was published. It stated that the Lockerbie bomber should be granted fresh leave to appeal.

The commission's verdict, after a three-year investigation, was to prove highly embarrassing for the Crown Office team which, until then, had been highly lauded for successfully bringing the case against Abdelbaset Ali Mohmed al Megrahi seven years ago.

The public statement said there were no fewer than six grounds on which Megrahi might have suffered a miscarriage of justice. Only four of those grounds, most of which focused on the unreliability of the Crown's key witness, were revealed to the public.

The Herald can now reveal the fifth: the Crown failed to disclose a top secret document with vital information about the timer which allegedly detonated the bomb.

Even now, the Crown is still refusing to show this highly confidential information to the defence, a decision all the more startling considering the current legal position on disclosure.

In 2005, the Judicial Committee of the Privy Council, the highest criminal appeal court, quashed the convictions of two Scots on the grounds that the Crown had failed to disclose vital documents and said that this breached the European Convention on Human Rights.

The judges ruled that the trials of James Holland, who had been convicted of two charges of assault and robbery, and Alvin Lee Sinclair, who was convicted of assault to severe injury, were in breach of Article 6 of the convention, the right to a fair trial. Statements, the judges said, should be made available as a "matter of course".

In Mr Holland's case, the prosecution was criticised for using a witness to identify the accused while in the dock, despite her failure to do so in a police line-up. There were also concerns raised about the Crown's failure to disclose outstanding criminal charges against witnesses.

In Mr Sinclair's case, the Crown failed to disclose a police statement which was inconsistent with the evidence given by a witness in court.

Since then, a number of high-profile convictions have been overturned on the grounds that important or inconsistent information was not revealed to the defence.

Last year, Stuart Gair was cleared of murdering a man in Glasgow after protesting his innocence for 17 years. Lord Abernethy, the judge, said a failure to disclose witness statements to his lawyers deprived the defence of a "powerful argument" on identification.

Next month, Nat Fraser, who was convicted of murdering his wife, Arlene, will have his appeal heard partly on such grounds.

However, two years on from the Privy Council's decision, legal experts say the problem has not been fully resolved and that miscarriages of justice are occurring as a result.

In Scotland, it has been up to the Lord Advocate and Crown Office to decide whether to allow the defence to see certain material, including information about previous convictions of witnesses.

The Crown Office has recently improved its system of disclosure and issued a practice statement setting out its aims to provide lists of witnesses within 14 days of first appearance and copies of witness statements within 28 days. The Crown is now obliged to disclose witness statements and previous convictions or outstanding charges against intended prosecution witnesses but it claims it also has to protect the human rights of witnesses.

In practice, lawyers say the system has been speeded up but key information is being omitted because the Crown says it is not in the "public interest". As one lawyer said: "It is as if the Crown is playing cards with a stacked deck."

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.

Interestingly, Lord Coulsfield, who chaired the review, was 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".

In relation to Lockerbie, the Crown is expected to argue that the top secret document relating to the timer cannot be disclosed for reasons of national security.

As Lord Coulsfield said, though, the ultimate question is whether Megrahi has had a fair trial and whether the Crown will release the documents for a fair appeal.