A US Senator has written to Gordon Brown, demanding the Lockerbie bomber be returned to prison in Scotland.

Charles Schumer said Britain should seek the "immediate" return of Abdelbaset al-Megrahi, three months after he was released from Greenock Prison.

The New York Democrat said Megrahi's early release on compassionate grounds was granted on the assumption that he had only three months to live.

He claimed that, as that time had elapsed, there was "speculation" that the severity of Megrahi's condition had been exaggerated.

Mr Schumer told the Prime Minister: "The bottom line is Megrahi should have never been released in the first place but it would be even more outrageous if he were to be able to live a long and free life after his release."

The senator said: "The victims of Pan Am Flight 103 didn't get a second chance at life and neither should Megrahi. Justice in this case was life in prison, no exceptions."

Megrahi, who has terminal prostate cancer, was freed on August 20 and flown home to Tripoli. He was serving a life sentence for the bombing of Pan Am flight 103 which claimed 270 lives in 1988.

His early release was greeted with anger by American relatives of those killed.

Announcing the decision to grant Megrahi compassionate early release, Scotland's Justice Secretary Kenny MacAskill said: "There are no fixed time limits but life expectancy of less than three months may be considered an appropriate period."

Later on Friday, a Scottish Government spokesman said Mr MacAskill's decision was based on recommendations by the parole board and prison governor, and was supported by a medical report.

The clinical assessment by the Scottish Prison Service health director was that a three-month life expectancy was "a reasonable estimate for this patient".

The spokesman said: "Mr al-Megrahi has terminal prostate cancer and was sent back to Libya to die. As Mr MacAskill said when he announced his decision, he may die sooner or may live longer, given the nature of his terminal disease."