Jack Straw, the former British foreign secretary, has also been asked to attend the hearing to be quizzed over his role in agreeing to a prison transfer agreement with Libya, where al-Megrahi is from.

'Highly unusual'

The requests have prompted deep unease in Britain, where the prospect of  government ministers answering to legislators from a foreign power is being viewed with concern.

Straw has not ruled out attending the hearing, but has said he will undertake consultations before replying as it would be "highly unusual" for a former minister to appear before such a committee.

He said there were "important issues of principle here which could affect UK governments of any party and which need carefully to be considered before I come to a final view".

Al-Megrahi was released on compassionate grounds last August after being diagnosed with terminal cancer. He was believed at the time to have three months to live, but is still alive and living in Libya.

He was serving a life sentence for the murders of 270 people killed when a US airliner exploded over the Scottish town of Lockerbie.

Influence investigated

US senators are investigating whether the terms of al-Megrahi's release could have been influenced by a multimillion dollar oil deal signed between British energy company BP and the Libyan government.

Tony Hayward, the chief executive of BP, has also been asked to attend the hearing to explain BPs role in lobbying for the bomber's release. The embattled boss of the energy giant is already feeling the ire of US politicians over the Gulf of Mexico oil spill and it is unclear whether he will attend.

David Cameron, the British prime minister, attempted earlier this week to defuse growing tensions between the two countries over the release, saying that the decision had been taken by the Scottish government rather than Westminster and was "wrong".

Barack Obama, the US president, said on Tuesday that Americans were "surprised, disappointed and angered" by the release.