The British people would find Tony Blair "unacceptable" as president of the European Council, Conservative leader David Cameron has warned.
Gordon Brown is lobbying for his predecessor to get the job, which would be created once the Lisbon Treaty is ratified by all EU member states.
Mr Cameron said this backing amounted to an "unelected prime minister pushing for an unelected president".
But Mr Brown told MPs the Tories wanted the UK to be at Europe's "fringes".
The prime minister lobbied for his predecessor to become president at last week's European Council meeting.
Of the 27 member states, only the Czech Republic has not ratified the treaty.
Mr Blair has not openly declared his intention to campaign for the role of president but the UK government has promised him its backing.
In the Commons, Mr Cameron told Mr Brown: "When considering your efforts to get Tony Blair this job, won't most people in Britain feel this: it is completely unacceptable to see an unelected prime minister pushing for an unelected president under a treaty that no one was allowed to vote for?"
In his speech to MPs, Mr Brown did not mention Mr Blair, but in response to questions he gave Mr Blair fresh backing and stressed that the leaders would not decide on who gets the role until the leaders meet once the Lisbon Treaty was ratified.
He also launched an attack on the Conservatives' new alliance in the European Parliament.
He said: "Once again we have shown that by acting not alone but together; by working not against our mainstream European partners, but with them; and by putting Britain not on the fringes of Europe, but in the heart of Europe - Britain will be stronger."
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