A former Libyan intelligence agent, Abdel Basset al-Megrahi, is serving a life sentence in Scotland for the 1988 bombing of an airliner over the town of Lockerbie, which killed 270 people.

Scotland's nationalist government, led by a party which favours independence from Britain, has accused the British government of making a deal to send Megrahi to Libya in return for oil contracts with British companies.

Straw said Megrahi could not be sent abroad without the permission of Scottish ministers.

"No deal has been done with Libya for the transfer of al-Megrahi," Straw wrote in the letter to the Herald newspaper.

"The decision on whether or not a prisoner detained in a Scottish prison is transferred to another country is a matter for Scottish ministers, not the United Kingdom Government.

"From the outset we have been clear with the Scottish Government about this fact and have not at any time sought to undermine due process in Scotland," he said.

Former Prime Minister Tony Blair signed an agreement on prisoner transfers with Libya last year, giving rise to suggestions he had made a deal to send Megrahi to Libya.

But Straw said the agreement was similar to ones Britain has with 98 other countries and did not single out Megrahi for any special treatment.

"The agreement will not provide for the transfer of any specific individual but will put in place a framework under which a prisoner may seek a transfer to serve his sentence in his own country," Straw wrote.

"The overriding principle of prisoner transfer arrangements is that a prisoner does not have any automatic right to transfer," he wrote.