Boris Johnson has accused David Cameron of being immoral for urging eurozone countries to pursue close political union.
Speaking in London on Tuesday morning, the mayor said the predictions of the euro-sceptics had turned out to be entirely correct and that the single currency had been "a calamitous project".
"I don’t understand why we continually urge the eurozone countries to go forward with this fiscal and political union, when we know in our hearts that it is anti-democratic and therefore intellectually and morally wrong," he said.
The prime minister and chancellor George Osborne have repeatedly called for the 17 eurozone countries within the 27 European Union form a tighter fiscal union in order to deal with the continent's economic crisis.
But Boris said: "The intention now is to make a bad situation worse. In the hope of reassuring the markets the eurozone countries are painfully composing a fiscal and political union that has no democratic legitimacy whatever.
"It cannot hope to have such legitimacy. This is a continent where there has been no single European political consciousness ... or sense of identity since the days of the Roman empire."
The Tory mayor of London called on Cameron to change his tactics with the aim of the UK having a looser relationship with the EU based on the single market.
"This pared down relationship is essential and deliverable, though you will hear a lot of huffing and puffing to the contrary.
"Our partners want us there, in the sense that they need us as a giant export market. They need the UK to be there at the council table, making the case for free markets and for common sense."