Mr Straw was forced into the denial after letters leaked to a Sunday newspaper appeared to show that he had backed away from efforts to stipulate that Abdelbaset Ali Mohmed al Megrahi should be exempt from a prisoner transfer agreement signed with Libya in 2007.

His comments were made as the father of one of the victims of the bombing of Pan Am 103 said it was time ‘‘to stop mulling over the why and wherefore of Megrahi’s release’’ and Nelson Mandela sent a letter of support to the Scottish Government.

Although ministers have rejected suggestions that Megrahi’s repatriation was linked to the UK’s commercial interests, suspicions have been fuelled by letters sent two years ago by Mr Straw to Scottish Justice Secretary Kenny MacAskill in which he says that, ‘‘in view of the overwhelming interests of the United Kingdom’’, he had agreed not to exclude Megrahi from a prisoner transfer agreement.

Mr Straw said: ‘‘The implication that, somehow or other, we have done some back-door deal in order to release Mr Megrahi is simply nonsense.

‘‘What makes this whole debate absurd now is that Mr Megrahi was not released under the prisoner transfer agreement.’’

Mr Straw admitted that in return for Libya abandoning its nuclear weapons programme there were moves to ‘‘establish wider relations including trade", but added: "the suggestion that at any stage there was some kind of back-door deal done over Mr Megrahi’s transfer because of trade is simply untrue’’.

Shadow foreign affairs minister David Lidington said it was ‘‘intolerable that we have to rely on leaks to find out what Labour ministers have been doing in our name.

‘‘We need a select committee inquiry so ministers and officials can be questioned about exactly what was said and done over relations with Libya.’’

LibDem MP Sir Menzies Campbell, a member of the Commons Foreign Affairs Select Committee, said: ‘‘Jack Straw’s intervention has simply muddied the waters.

‘‘We need a full and frank comprehensive statement about the extent to which Mr Megrahi’s fate may have featured in any trade negotiations between the United Kingdom and Libya.’’

First Minister Alex Salmond said it was a matter of record that his administration had opposed the prisoner transfer agreement.

‘‘We didn’t think that the Lockerbie decision should be linked to trade or oil decisions by anyone who looked at the coincidence that the prisoner transfer agreement was being negotiated at the same time as commercial contracts,’’ he said.

The Scottish Government has pledged to publish all correspondence connected to Megrahi’s release before a Holyrood debate on Wednesday, and Mr Straw confirmed that Westminster would ‘‘shortly’’ be releasing its correspondence.

Nelson Mandela played a central role in facilitating the handover of Megrahi to the United Nations so he could stand trial under Scottish law in the Netherlands, and subsequently visited him in Barlinnie Prison in Glasgow.

His backing emerged in a letter sent by Professor Jake Gerwel, chairperson of the Mandela Foundation.

He said: ‘‘Mr Mandela sincerely appreciates the decision to release Mr al Megrahi on compassionate grounds.

‘‘His interest and involvement continued after the trial after visiting Mr al Megrahi in prison.

‘‘The decision to release him now, and allow him to return to Libya, is one which is therefore in line with his wishes.’’

Dr Jim Swire, whose daughter Flora was killed in the atrocity, called on the authorities in Scotland to ‘‘take responsibility’’ for reviewing Megrahi’s conviction.

In a letter to the media, Dr Swire said he was ‘‘delighted’’ that Megrahi, who has terminal prostate cancer, had been freed.

He said: ‘‘Let us stop mulling over the why and wherefore of Megrahi’s release.

‘‘The public’s knowledge of the shifty dealings surrounding the prisoner transfer agreement should help to swell demand for objective assessment of the Megrahi case.’’

He continued: ‘‘Let us turn our attention now, please, at last to the question of why we, the relatives, have been denied our rights to know who really murdered their families, and why those precious lives were not protected.’’