UK opposition leader David Cameron on why he does not want President Blair
Gordon Brown has said he would be "very happy" to support a bid by his predecessor Tony Blair to be the first president of the European Council.
But the prime minister told MPs the post did not yet exist as the Lisbon Treaty creating it had not become law.
The BBC understands Mr Brown will put Mr Blair's case to other EU leaders in Brussels later this week after previously denying it would do so.
But Luxembourg PM Jean-Claude Juncker has also declared himself a candidate.
Italian Prime Minister Silivio Berlusconi is Mr Blair's most vocal backer among EU leaders.
'Must find consensus'
But his foreign minister Franco Frattini, who had previously indicated backing for a Blair presidency, said that "the entry into the fray of Mr Juncker and why not (Dutch Prime Minister Jan-Peter) Balkenende changes the picture."
He added: "We cannot imagine a divided Europe. We must find a consensus."
Mr Brown was quizzed about the proposed new post at prime minister's question time in the Commons.
He told MPs: "If this position is to be created - and the EU treaty is not yet through - and the former prime minister Tony Blair comes forward as a candidate, we will be very happy to support him."
We want to bring European countries closer together, and I think if someone is from a smaller country these people are more sensitive
On Tuesday, Conservative leader David Cameron, who opposes the treaty and the new post, said: "We don't support Tony Blair in that role even if there is a president."
Meanwhile, it is understood that Lib Dem leader Nick Clegg will on Wednesday publicly declare his opposition to Mr Blair's predicted candidacy.
Nigel Farage, leader of the UK Independence Party, which wants the UK to leave the European Union, said he backed Mr Blair's bid: "If Mr Blair does become Mr President then nothing better will bring home to the British people the truth about the EU, that it is not, in fact, run by democratically-elected representatives or those who we can elect and remove."
Mr Blair has refused to state publicly that he is in the running for the proposed post of president of the European Council although there have been widespread reports that people have been lobbying on his behalf behind the scenes.
He is now under pressure to come out and declare that he does want the job amid opposition to his candidacy among some smaller European states.
Belgium, the Netherlands and Luxembourg are reportedly among those that believe the president should come from a country that uses the euro and is part of the border-free Schengen Agreement.
There are reports French President Nicolas Sarkozy is cooling on the idea of a Blair presidency and it is not known whom German Chancellor Angela Merkel would support.
But Ms Merkel's new coalition partners - the Free Democrats, who control the country's foreign ministry - have expressed doubts about Mr Blair.
The party's chief whip, Joerg van Essen, told the BBC: "We have known Tony Blair for a very long time but I must admit there is a sympathy in my party for candidates from a smaller country."
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