Dr Richard Simpson said that medical reports show there is "significant doubt" that Abdelbaset Ali Mohmed al Megrahi will die within the next three months, Telegraph reported.
The Labour MSP accused Kenny MacAskill, the Scottish justice minister, of failing to conduct sufficient checks before deciding to release the terminally-ill bomber last week.
This attack was echoed by the Tories, who said that the most recent medical consensus was Megrahi would live eight months, too long to be eligible for compassionate release.
The row broke out as Gordon Brown finally ended his silence on the controversy, but refused to say whether he agreed with Mr MacAskill's decision.
The Prime Minister stressed he had "no role" in the release and he was "angry and repulsed" at the hero's welcome that greeted Megrahi on his return to Libya.
A storm of international condemnation has met Mr MacAskill's ruling last week to release Megrahi, who is suffering from prostate cancer, on compassionate grounds.
Scottish Prison Service (SPS) guidelines suggest that inmates are only freed if they have less than three months to live.
However, Dr Simpson, who specialised in prostate disease research, said: "It is clear to me from the medical reports and the opinion of the specialists that Megrahi could live for many more months.
"Kenny MacAskill released him apparently on the advice of just one doctor whose status is not clear and who is not named."
Dr Simpson, a former member of the British Association of Urological Surgeons' prostate cancer working group, said the minister should have sought a second opinion from a specialist in palliative care.
A health assessment compiled by a SPS medical officer for Mr MacAskill, states that last autumn Megrahi was given between 18 months and two years to live.
However, a range of specialists concluded in June and July this year that his condition had deteriorated over the intervening 10 months and the lower end of this scale was more likely.
Megrahi's life expectancy was no longer deemed to be "many months" but the report concluded: "Whether or not prognosis is more or less than three months, no specialist would be 'willing to say'."
However, his personal physician said his condition "declined significantly" between July 26 and August 3 and a life expectancy of three months was deemed a "reasonable estimate".
Bill Aitken, Scottish Tory justice spokesman, said: "In June and July, there was a consensus on prognosis of eight months. Where is that consensus now?
"We only have the opinion of one anonymous individual - not the range of medical experts promised."
The Scottish Executive admitted that Megrahi could live longer than three months, but insisted this prognosis was supported by a wide range of medical experts.
A spokesman said Mr MacAskill had based his decision on a report by the SPS director of health and care, who had access to all Megrahi's medical records.
He said this contained a "clear" clinical assessment that the bomber's life expectancy was three months or less.
Gordon Brown argued that, because Scottish judicial issues are devolved, his Government "could not interfere and had no control over the final outcome".
But he added: "I was both angry and I was repulsed by the reception that a convicted bomber guilty of a huge terrorist crime received on his return to Libya."
The Prime Minister had pleaded in a personal letter to Colonel Gaddafi, the Libyan leader, for restraint.
Speaking at a Downing Street press conference with Binyamin Netanyahu, the Israeli prime minister, he said he did not think the affair would harm relations with other countries in the fight against terrorism.
But William Hague, the shadow foreign secretary, said: "He still hasn't said whether or not he was happy with the decision, a decision of huge public concern in this country, of immense international importance, which President Obama and so many other people in other countries have commented on.
"I think it is part of a continuing failure of leadership that he is unable to say that he will defend the decision or that he was not happy with it. And that is not what we expect from the Prime Minister of Britain."
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