Boris Johnson has intervened in startling fashion in the coalition Government's controversial cuts to housing benefit, incensing Downing Street by suggesting that the reforms could cause "Kosovo-style social cleansing" of the poor from London.
The extravagantly spoken London mayor remains engaged in difficult talks with Iain Duncan Smith, the Work and Pensions Secretary, over ways to soften the impact of the reforms on the capital. He fears that thousands of families could be driven from the centre of the city and forced to live in the suburbs or temporary accommodation, including bed and breakfasts.
But he appeared to raise the stakes today in launching a broadside against David Cameron, voicing the growing unease of MPs with urban seats that the cuts will inevitably lead to families having to relocate.
"The last thing we want to have in our city is a situation such as Paris where the less well-off are pushed out to the suburbs," Mr Johnson said in an interview with the BBC. "I'll emphatically resist any attempt to recreate a London where the rich and poor cannot live together.
"We will not accept any kind of Kosovo-style social cleansing of London. On my watch, you are not going to see thousands of families evicted from the place where they have been living and have put down roots."
His remarks were immediately condemned by Downing Street, with the Prime Minister's official spokesman saying that Mr Cameron did not agree "with what Boris Johnson has said or indeed the way he said it".
The Lib Dem Deputy Prime Minister Nick Clegg echoed the sentiment: “In London there are hotspots of very high property prices which create particular dilemmas and difficulties. But I disagree with what Boris Johnson has said on the policy and I certainly and very strongly disagree with the way in which he has expressed his views.”
The Government estimates that around 21,000 homes will be affected by the changes, with 17,000 of them in London. Under the reforms, housing benefit will be limited to £250 a week for a one-bedroom home, or up to £400 a week for a four-bedroom property. Claims will be cut by 10 per cent for those who have received Jobseeker's Allowance for more than a year.
The spat is the latest chapter in the difficult relationship between the two senior Tories, who were contemporaries at both Eton and Oxford. Since Mr Johnson's successful mayoral bid, he has challenged the Prime Minister over a number of key issues.
While he is currently tolerated within No 10, with advisers to the Prime Minister sticking to the line that "Boris is Boris", patience is said to be wearing thin. Last year, Mr Johnson derailed his leader's hopes of a smooth party conference by backing calls for a referendum on the Lisbon Treaty.
Since then, the London mayor has leapt to the defence of bankers after Mr Cameron has condemned them for their role in causing the financial crisis and called for an end to the 50p tax on those earning more than £150,000 - a Labour policy which the Prime Minister has opted to keep. He also said Mr Cameron's claim that Britain was "broken" as "piffle".
Many see Mr Johnson, who is to run for a second term as London mayor, as the only credible challenger to Mr Cameron. Senior Tories and Liberal Democrats were defending the Prime Minister last night. Sir George Young, the Leader of the Commons, said the mayor's warning was not "appropriate". Vince Cable, the Liberal Democrat Business Secretary, criticised Mr Johnson's use of "inflammatory language on a difficult and sensitive issue".
Later in the day, Mr Johnson moved to distance himself from his own remarks, trying to defuse the row through a statement suggesting that his words had been taken "out of context".
"My consistent position has been that the Government is absolutely right to reform the housing benefit system which has become completely unsustainable," he said. "I do not agree with the wild accusations from defenders of the current system that reform will lead to social cleansing.
"But the point I was making this morning is that London has specific needs due to the exceptional way in which the housing market works in the capital and it is my job as Mayor to make the Government aware of these."