The British government decided it was in the overwhelming interests of the United Kingdom to make Abdelbaset Ali Mohmed al-Megrahi, the Lockerbie bomber, eligible for return to Libya, leaked ministerial letters reveal.

Gordon Brown`s government made the decision after discussions between Libya and BP over a multi-million-pound oil exploration deal had hit difficulties. These were resolved soon afterwards.

The letters were sent two years ago by Jack Straw, the justice secretary, to Kenny MacAskill, his counterpart in Scotland, who has been widely criticised for taking the formal decision to permit Megrahi`s release.

Within six weeks of the government climbdown, Libya had ratified the BP deal. The prisoner transfer agreement was finalised in May this year, leading to Libya formally applying for Megrahi to be transferred to its custody.

Saif Gadaffi, the colonel`s son, has insisted that negotiation over the release of Megrahi was linked with the BP oil deal: The fight to get the transfer agreement lasted a long time and was very political, but I want to make clear that we didn`t mention Mr Megrahi.