Downing Street says claims two senior civil servants are lobbying behind closed doors for Tony Blair to become EU president are nothing but "speculation".
The Guardian newspaper reported that Gordon Brown's senior Europe adviser John Cunliffe and EU ambassador Kim Darroch were asked to take soundings.
No 10 has refused to confirm the claim. But a diplomatic source told the Guardian that "Kim is talking to everyone".
"There wouldn't be any sense in the prime minister doing that given that (a), we haven't got the treaty ratified, and (b), Tony Blair hasn't decided whether he will be a candidate," the prime minister's spokesman said.
Mr Blair has himself been reticent about his potential candidacy. He has not confirmed an interest in assuming a prominent role in Europe, reportedly causing frustration among Mr Brown's staff that they are unappreciated.
Foreign secretary David Miliband, openly backing Mr Blair's undeclared candidacy, told the BBC yesterday: "This is a time when a strong European voice is more needed than ever, it's not a time for shy retiring violets."
The former prime minister has not been inactive since he left Downing Street in 2007, working most prominently as a Middle East peace envoy.
He now finds himself a potential frontrunner for the European Commission presidency.
This so-called 'president of Europe' role would come into effect if the Lisbon treaty is ratified.
Speaking in his monthly press conference this morning, Conservative leader David Cameron made clear he did not support Mr Blair in the potential role. He said he preferred a "chairmanic" president to an "all-singing, all-dancing, all-acting" figurehead.
"I can see what sort of president Tony Blair would be," he added.
Meanwhile, those on the left of the plitical spectrum were also attacking the suggestion.
South East Euro-MP and Green party leader Caroline Lucas said the suggestion Mr Blair should be made president showed "just how detached Labour ministers have now become from widespread public feeling".
"The suggestion is nothing short of absurd," she added.
"Can there be anyone outside of the government who really believes that the man who snubbed the EU over Iraq and joined with George W. Bush to lead us into an illegal war is now the right person to become the EU's first council president?"