Tory leader David Cameron today stepped up calls for Gordon Brown to express an opinion on Scotland's decision to release the Lockerbie bomber.
Mr Cameron said the Prime Minister had said he was "repulsed" by the welcome received by Abdelbaset Ali al Megrahi in Tripoli - but "so was everyone".
In a letter to Conservative supporters, Mr Cameron also said Mr Brown should set out the "dealings" the Government had had with Libya on the issue.
He wrote: "Most importantly of all, what is Gordon Brown's opinion of the decision to return Mr al Megrahi on compassionate grounds?
"I have made my view clear. I think it was wrong. I see no justice in affording mercy to someone who showed no mercy to his victims.
"Others have expressed similar opinions too.
"But what does the Prime Minister think? It's not good enough hiding behind the cloak of constitutional convention and saying this is a devolved matter.
"On a matter of international importance, which has damaged our reputation abroad and undermined relations with our foremost ally, we need to know what the Prime Minister thinks."
On Tuesday, Mr Brown said he had been "angry" and "repulsed" by the jubilant scenes in Tripoli but refused to say whether he supported the move by the Scottish Government.
He added that he had made it "absolutely clear" to Libyan leader Colonel Gaddafi that the decision on Megrahi's release was in the hands of Scotland alone.
In the letter, Mr Cameron said: "All (Mr Brown) has said is that he was "angry" and "repulsed" by the scenes in Tripoli.
"Of course he was. So was everyone. Seeing a convicted murderer being treated like a returning hero was disgusting, especially for those who lost loved ones in the tragedy."
He said Conservative MPs had been putting down parliamentary questions to find out what "dealings" ministers had had with Libya on Megrahi's release.