A cancer specialist who has visited the man convicted of the Lockerbie bombing says the Libyan has an "aggressive" form of prostate cancer that is no longer responding to treatment.

With Kenny MacAskill set to release Abdelbaset Ali Mohmed al Megrahi today, the assessment given by Professor Karol Sikora goes some way to explain why the Justice Secretary may have reached the decision he has.

Dr Sikora, who visited Megrahi in prison, had called for an "urgent" decision on the future of the Lockerbie bombing prisoner before his condition worsens further.

"We believe he has only a very short period of time to live," said Dr Sikora, who assessed Megrahi last month.

His call came as US pressure mounted on the Scottish Government to keep Megrahi behind bars in Greenock prison.

Hillary Clinton, the US Secretary of State, has already telephoned Mr MacAskill to say Megrahi should serve out his sentence in Scotland, and she repeated that view yesterday.

She told US reporters: "I just think it is absolutely wrong to release someone who has been imprisoned based on the evidence about his involvement in such a horrendous crime."

However, ministers have said the decision on whether to release Megrahi will be based on judicial grounds and the medical evidence, rather than politics.

Dr Sikora, the medical dir- ector of the independent cancer care network CancerPartnersUK, said he was asked by the Libyan government to provide an independent medical assessment of Megrahi.

He visited him on July 28 with Professor Ibrahim Sheriff from the Tripoli medical centre, Libya and Dr Abdulrahman Swessi, Libya's consul-general in Scotland.

"We were shown great courtesy by the prison staff and especially the prison doctor with whom we discussed medical details and reviewed records," he said.

"Despite rumours in the media to the contrary, Mr Al Megrahi has an aggressive form of prostate cancer that has spread widely.

"Although he initially responded to treatment, this is no longer working. We believe he has only a very short period of time to live."

He added: "We found him to be a highly intelligent, well-educated and deeply religious person who wishes to spend his last few weeks with his wife and five children.

"We believe an urgent decision on his future is needed before any further medical deterioration takes place."

Megrahi successfully dropped his appeal against conviction on Tuesday amid mounting speculation he could be moved to Libya within days on compassionate grounds.

Earlier this week three appeal judges in Edinburgh heard Megrahi's condition had worsened "very considerably" in recent days. There is concern that he would not survive the fasting of Ramadan while in prison.

Mr MacAskill has been forced to decide whether Megrahi, convicted in 2001 of the 1988 airliner bombing which killed 270, should return to Libya - either as a serving prisoner under a transfer agreement, or as a free man on compassionate release.

John Swinney, Scotland's Finance Secretary, said: "He's gone to very significant lengths to listen to the families of those who lost loved ones in the atrocity and he's listened to the views of the various governments involved."