Four US senators have written a letter calling on Britain to investigate the circumstances surrounding last year's release of the Lockerbie bombing.

Terminally-ill Abdelbaset Al Megrahi was freed from prison and allowed to return to Libya on compassionate grounds in August last year.

New York Democratic Senators Kirsten Gillibrand and Charles Schumer and New Jersey Senators Robert Menendez and Frank Lautenberg have written to the UK's ambassador in Washington.

The letter follows British cancer expert Professor Karol Sikora's admission to a newspaper earlier this week that Megrahi's survival past a three-month prognosis was "embarrassing".

The medical director of CancerPartners UK, who compiled a report on Megrahi's condition for the Libyan government before he was freed, also said: "There was always a chance that he would live for 10 years, 20 years... But it's very unusual."

Senator Lautenberg said in a statement on his website: "I was outraged when Mr Al Megrahi was released from prison and it is sickening that this convicted terrorist may have been released under false pretences."

Senator Gillibrand said: "The latest reports are extremely troubling. We in the United States opposed the decision by the Scottish court to release Abdelbaset Al Megrahi.

"Justice was not served and the families of Pan Am Flight 103 were forced to relive the horrific loss of their loved ones. Megrahi should serve his full sentence and spend the rest of his days in prison."

Prof Sikora said: "It is embarrassing that he's gone on for so long."

He denied he had come under any pressure from the Libyan authorities but said he had seen other patients with the same symptoms as Megrahi who had lived longer than three months.

He said: "There's a 50% chance that he would die in three months, but there was also a 50% chance that he would live longer."

He also said: "It was clear that three months was what they were aiming for. Three months was the critical point. On the balance of probabilities, I felt I could sort of justify (that)."
Megrahi is the only man to have been convicted of the Lockerbie atrocity, which saw 270 people killed in the bombing of Pan Am flight 103 on December 21, 1988.

His release from Greenock prison was met with anger, especially among victims' relatives and politicians in the US.

Megrahi was greeted by cheering crowds in Tripoli after flying home from Scotland, scenes which prompted widespread outrage in Britain and the US.

Scottish Justice Secretary Kenny MacAskill cited the three-month prognosis when he released the convicted bomber on August 20 last year.

Prof Sikora was asked by the Libyan government to provide an independent medical assessment of Megrahi last summer. He visited him on July 28, 2009 with Professor Ibrahim Sherif from the Tripoli medical centre, Libya, and Dr Abdulrahman Swessi, Libya's consul general in Scotland.