Megrahi received an ecstatic welcome when he returned to Libya
A suggestion that the freeing of train robber Ronnie Biggs was linked to the impending release of the Lockerbie bomber has been angrily denied.
Former Tory leader Michael Howard has written to Justice Secretary Jack Straw asking if the releases of Abdelbaset Ali al-Megrahi and Biggs were linked.
Both were freed on compassionate grounds last month.
But Mr Straw's spokesman said there was no connection and that Mr Howard's comments were "extremely offensive".
Mr Straw decided on 8 August to free Biggs, citing medical evidence that his condition had deteriorated and he was not expected to recover.
'Not satisfactorily explained'
On 20 August, the Scottish government - responsible for justice matters in the country - decided to release Megrahi.
He was freed eight years into a life sentence imposed for his part in the bombing of a Pan Am flight over Lockerbie in December 1988, which killed 270 people.
Mr Howard's letter suggests Mr Straw overturned a previous decision not to free Biggs because the justice secretary knew Megrahi was about to be returned to Libya by the Scottish government.
The former Conservative leader, who was home secretary during the 1990s, wrote that the sequence of events made it "very hard" to avoid the inference that the U-turn in the Biggs case had been influenced by Mr Straw's "knowledge of the likely decision of the Scottish government in the Megrahi case".
He added: "If this was indeed true, it would amount to a very serious breach of the principles you were, by law, required to apply in the Biggs case."
Mr Howard said Mr Straw had originally refused to release Biggs because he had shown no remorse for his crimes, but he reversed the decision a month later.
"The reason for this change of mind on your part has never been satisfactorily explained," Mr Howard wrote.
"If Megrahi had been released on compassionate grounds so that he did not die in prison, while you adhered to your original decision in the Biggs case, the contrast would, of course, have been stark and obvious, particularly since Megrahi - like Biggs - had refused to express remorse for the crime of which he had been convicted."
But a spokesman for Mr Straw said there was "no connection at all between the two cases" and dismissed the claims as "ridiculous".
He said: "The idea of any connection between the Biggs case and the release of Megrahi is preposterous, untrue, absurd and offensive.
"As Jack said at the time of his decision, Ronnie Biggs was released on compassionate grounds after he considered the medical evidence against well-established criteria, specifically whether death was likely to occur soon and whether the prisoner was bed-ridden or severely incapacitated.
"These were different criteria to Mr Biggs' application for parole where the justice secretary refused parole, principally because Mr Biggs had shown no remorse for his crimes nor respect for the punishments given to him.
"Megrahi was released by the Scottish Executive. That was their decision and their decision alone. Jack became aware of al-Megrahi's release when he saw the news on the BBC website.
"There is no connection at all between the two cases. The suggestions being made by Mr Howard are ridiculous, entirely without foundation and extremely offensive."