Special advocates would mark another twist in the often secretive but invariably controversial Lockerbie appeal.

After a European Court of Human Rights decision in 2003, the House of Lords ruled that in "exceptional circumstances" special counsel should be appointed to ensure the judge did not just hear the views of the prosecution in Public Interest Immunity (PII) hearings.

The aim was, in part, to protect the human rights of the defendant (as referred to in England and Wales) and a list of special, security-cleared counsel was created, which has been used to deal with certain terrorism cases where previously there had been no defence representation.

There is a long history of PII certificates granted south of the border, with varying degrees of success.

The problem for the defence team involved is that they do not know what is contained in the document they are arguing to see. They are, as one lawyer put it, "playing blind against a stacked deck".

The Scott Report, investigating the infamous Arms to Iraq deals involving British companies in the 1980s, revealed inappropriate and often gross misuse of the certificates.

Sir Richard Scott, then a Lord Justice of Appeal, found that while some of the certificates protected files containing potentially sensitive intelligence material, many were simply internal communications: certificates were intended to protect the ministers and civil servants.

Special counsel have been used in ongoing terrorism trials south of the border to varying effect. But experts argue that in relation to the Lockerbie appeal - relating to an event some 20 years ago - they should not be necessary and the defence team should be allowed to represent themselves.

"It is very very difficult for a special advocate to argue the importance of disclosing a particular document if they are not aware of the intricacies and nature of defence tactics," said Professor Robert Black, one of the architects of the trial at Camp Zeist.

"The rationale for this in England and Wales is usually that disclosing to the accused or to his defence lawyers might prejudice ongoing inquiries, but there are no ongoing inquiries on Lockerbie. It took place 20 years ago so this is an entirely different scenario. In very exceptional circumstances PII hearings have been held behind closed doors, but I am not aware of any similar case in Scotland where the serving advocates would have been evicted in this way."

The degree to which information is fully disclosed by the Crown to the defence has long been a thorny subject in Scottish law.

Traditionally, the decision about whether to share some material, including inconsistent police statements and information about witnesses' previous convictions, has lain with the Lord Advocate.

South of the border, rules on disclosure have been set in statute to clarify the position - a move which Scotland will shortly follow. In general, the Lords' ruling made it clear that the starting point is the "Golden Rule", that any material which weakens the prosecution case or strengthens that of the defence should be disclosed.

In this case, the Scottish Criminal Cases Review Commission has already said in its decision last summer that the Crown's failure to disclose this document to the original defence team could potentially constitute a miscarriage of justice.

After the recent review of disclosure in Scotland by retired High Court judge Lord Coulsfield, Justice Secretary Kenny MacAskill revealed last month Scotland would also be putting the issue on a statutory footing.

He also suggested the introduction of a system of PII hearings to achieve a "balance" between protecting sensitive or confidential information and the requirement to disclose.

In the case of the Lockerbie appeal, the needs of the relatives to have a full and public hearing and the rights of the accused to have a fair appeal have to be weighed up against the arguments of the prosecution.