Nick Clegg has urged critics not to exploit the eurozone crisis to foster "xenophobia, chauvinism and polarisation" as doubts were cast on political efforts to save the single currency.
Credit rating agency Fitch said it believed that a comprehensive solution was now "technically and politically beyond reach" despite last week's agreement on a rescue package.
And Tory former chancellor Lord Lamont warned that time was running out to develop sufficiently strong measures to prevent the euro from an fatal "explosion".
In moves that will be closely watched by markets, Fitch put the ratings of six countries - Belgium, Spain, Slovenia, Italy, Ireland and Cyprus - under review.
And it dropped France's economic outlook from stable to negative - meaning it believes the country is more likely than not to lose its cherished AAA credit rating within two years.
Paris was delivered a diplomatic slap by the Deputy Prime Minister for "simply unacceptable" attacks on the UK economy by a series of senior politicians and officials.
Prime minister Francois Fillon and its central bank chief had questioned why the UK was not facing a downgrade despite being "even more indebted than us and carrying a bigger deficit".
As cross-Channel relations spiralled downwards, fuelled by Prime Minister David Cameron's EU treaty veto at last week's summit, Mr Clegg told the French PM to "calm the rhetoric".
He was said to have secured such a promise and an agreement to discuss economic co-operation.
Fitch said it believed France was the "most exposed to a further intensification of the crisis" among AAA countries, partly because of its place in the euro bailout fund. Its rating is also under review by another leading ratings agency - Standard and Poor's.
