Mr Cameron said the decision had been "wrong"
Conservative leader David Cameron has condemned the release of Lockerbie bomber Abdelbaset Ali al-Megrahi as "completely nonsensical".
Megrahi, convicted of the murder of 270 people, was freed on compassionate grounds as he is dying of cancer.
Mr Cameron said Scottish justice minister Kenny MacAskill had made a "very bad" judgement.
But Chancellor Alistair Darling said it was right that the Scottish government had reached its own decision.
Megrahi, a 57-year-old Libyan, has served eight years of a life sentence imposed for the 1988 atrocity - in which a plane was blown up above the town of Lockerbie - and is returning to his home country.
Mr Cameron said: "I think this is wrong and it's the product of some completely nonsensical thinking in my view.
"If there's a view that the conviction is in some way unsafe, then the proper process is an appeal and the presentation of new evidence.
"But if this is about genuine release on compassionate grounds, I think it is wrong.
"This man was convicted of murdering 270 people. He showed no compassion to them. They weren't allowed to go home and die with their relatives in their own bed. And I think this is a very bad decision."
The matter was dealt with by the Scottish government, under the terms of UK devolution.
Mr MacAskill, of the ruling Scottish National Party, had been under intense pressure from the US government to keep Megrahi behind bars, with Secretary of State Hillary Clinton saying his release would be "absolutely wrong".
In a BBC interview ahead of the announcement of the release, Mr Darling, who is standing in for Prime Minister Gordon Brown, who is on holiday, said: "You either devolve the responsibility for criminal justice or you don't.
"And I bet you, if I'd been saying what Kenny MacAskill ought to do, then many people would've cried foul and said 'This is nothing to do with you. Why are you interfering?'
"I think we just respect the fact that it's been devolved. It's Kenny MacAskill's decision, that of the Scottish government.
"They have to make that decision. They have to justify whatever that decision is."
But ex-Labour MP Tam Dalyell, a former father of the House of Commons, said: "Mr MacAskill... has arrived at the right decision on compassionate grounds.
"I do not accept his endorsement of the guilt of Mr Megrahi, whom I continue to believe had nothing whatsoever to do with the crime of Lockerbie."