Jack Straw yesterday refused to say whether he will allow letters between himself and the Scottish Government to be made public.

Following the revelation in The Herald yesterday that First Minister Alex Salmond has written to the Lord Chancellor, requesting the publication of their correspondence, Mr Straw last night refused to respond at this stage.

Earlier this week, in a letter to The Herald, Mr Straw insisted that Scottish ministers would have the final say on whether to transfer the Lockerbie bomber, following claims that he was a pawn in a recent £450m oil deal with Libya.

However, his comments unleashed renewed criticism from the Scottish Government for failing to explain why Westminster had not obtained an order specifically excluding Abdelbaset Ali Mohmed al Megrahi from the infamous "deal in the desert" made by Tony Blair last year.

Professor Robert Black, one of the architects of the Lockerbie trial at Camp Zeist, yesterday accused the Westminster Government and former Prime Minister Blair of being "disingenuous" and dishonest about the prisoner transfer agreement.

He said: "When the UK Foreign Office entered into negotiations with Libya for a reciprocal prisoner transfer agreement, both sides were perfectly well aware that the only Libyan in a British jail whom the Libyans had the slightest concern about was Megrahi. The Libyan negotiators believed, and were known to believe, that the agreement they were drafting would cover Megrahi.

"The London government did not have the courtesy to inform the Scottish Government about these negotiations and later said the agreement would not cover Megrahi. This was at best disingenuous and, at worst, an outright lie."

It also came to light yesterday that the prisoner transfer agreement has not yet been officially signed off, and Mr Salmond is now pushing for Mr Straw to go back to Libya to persuade them to incorporate a clause specifically excluding Megrahi.

A source close to the First Minister said: "Mr Straw needs to go back to Libya and ensure that what they promised comes to pass. The prisoner transfer agreement should include a clear and specific exemption in relation to the man convicted of the Lockerbie bombing. This was the position they signed up to."

A spokesman for the Lord Chancellor said: "Mr Straw will respond in due course. The prisoner transfer agreement has not yet been concluded."

Sir Menzies Campbell, former UK leader of the LibDems, said Mr Salmond should clarify his request for the agreement to be renegotiated.

"There is no evidence to suggest Libya intends to renegotiate the agreement," he said. "The prospect of that seems remote but Mr Salmond should make it clear that is what he is asking for."

Whitehall has repeatedly denied that Megrahi, who is serving 27 years in Greenock Prison for the attack, was part of the arrangement signed by the former Prime Minister. However, Libyan officials and lawyers have maintained that Megrahi was a key part of the discussions, which have been ongoing since 2005.

The agreement means any Libyan serving their sentence in the UK, and who has no pending appeal, could return home. However, under the law, those serving sentences in Scottish prisons can be moved only with the permission of Scottish ministers.

Westminster officials said Britain had prisoner transfer agreements with more than 100 countries and that Scottish ministers would have the final say in the Lockerbie case.

They argued that none of the other agreements contained exclusion clauses but the news last month that Libya had ratified a £450m exploration contract with BP reignited suspicions about the details of the agreement.

Fall-out from deal in the desert' How did the row about the potential prisoner transfer of Megrahi start this week? Jack Straw wrote to The Herald to clarify the Westminster Government's position. He said that any decision to move Megrahi lay in the hands of Scottish ministers.

Why did he write the letter? He was responding to a letter published in the paper from Jim Swire, whose daughter Flora died in the tragedy, which raised concerns about what Tony Blair may have promised Colonel Gaddafi during their "deal in the desert" in which the two leaders agreed on reciprocal extradition and transfer of prisoners.

What is the prisoner transfer agreement? A draft "Memorandum of Understanding on the pursuit of agreements on judicial co-operation" was signed by the British and Libyan governments in June last year when Mr Blair was visiting Colonel Gaddafi. It referred to "extradition and prisoner transfer". No prisoner is named, but the memorandum states: "The UK government will seek to obtain the agreement of all three jurisdictions within the UK in each of these cases." The final agreement has not yet been signed.

Why does the Scottish Government believe it is important for Megrahi to be specifically excluded from the agreement made between Libya and Westminster? The original international agreement, which allowed Megrahi to be put on trial at a special court at Camp Zeist, also stated that any person convicted would serve their full sentence in Scotland. Alex Salmond was not told about the "deal in the desert" until after the new agreement had been drafted, despite the fact Megrahi is held in a Scottish jail. Ensuring the Libyan serves the full sentence here, he believes, is vital to maintaining the integrity of Scots law. If Megrahi fails to win his current appeal, unless he is excluded from the agreement, he could push for judicial review of a decision to hold him in a Scottish prison, a fact Mr Straw has acknowledged in a private letter to Mr Salmond.

Why could he make a case for judicial review? Judicial review is a High Court procedure for challenging administrative decisions of public bodies. If Justice Secretary Kenny MacAskill were to refuse a transfer request, Megrahi could challenge that decision in the courts. He could, for example, argue that all other Libyan prisoners in the UK had been moved and that the decision to keep him was unfair.

Why could Mr Straw not secure an exemption for Megrahi? Westminster officials argue that Libya turned down the request and point out there are no exclusion clauses in similar agreements with at least 100 other countries. They argue it would be almost impossible for Megrahi to win a judicial review.

What happens next? Next Wednesday, Megrahi's legal team will be in court arguing for the Crown to disclose a top secret document. Depending on the results of this hearing and several other procedural hearings, the appeal itself, if it goes ahead, could be heard later this year.