The British government decided it was “in the overwhelming interests of the United Kingdom” to make Abdel Basset Ali 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 critizised for taking the formal decision to permit Megrahi’s release.

The correspondence makes it plain that the key decision to include Megrahi in a deal with Libya to allow prisoners to return home was, in fact, taken in London for British national interests.

Edward Davey, the Liberal Democrat foreign affairs spokesman, said, “This is the strongest evidence yet that the British government has been involved for a long time in talks over al-Megrahi in which commercial considerations have been central to their thinking.”

Two letters dated five months apart show that Straw initially intended to exclude Megrahi from a prisoner transfer agreement with Colonel Muammar al-Qaddafi, under which British and Libyan prisoners could serve out their sentences in their home country.

In a letter dated July 26, 2007, Straw said he favoured an option to leave out Megrahi by stipulating that any prisoners convicted before a specified date would not be considered for transfer.

Downing Street had also said Megrahi would not be included under the agreement.

Straw then switched his position as Libya used its deal with BP as a bargaining chip to insist the Lockerbie bomber was included.

The exploration deal for oil and gas, potentially worth over $24 billion, was announced in May 2007. Six months later the agreement was still waiting to be ratified.

On December 19, 2007, Straw wrote to MacAskill announcing that the U.K. government was abandoning its attempt to exclude Megrahi from the prisoner transfer agreement, citing the national interest.

In a letter leaked by a Whitehall source, he wrote: “I had previously accepted the importance of the al-Megrahi issue to Scotland and said I would try to get an exclusion for him on the face of the agreement. I have not been able to secure an explicit exclusion.

Click here for the full report from the London Times.