Boris Johnson has criticised Labour's plans to abolish private schools as being expensive and a "pointless attack" on the education system.
The Eton-educated prime minister accused Jeremy Corbyn of "unbelievable hypocrisy" over the proposal backed by Labour members at its conference.
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If Labour were to form a government following a general election, it would seek to "integrate" fee-paying schools into the state sector.
The party would strip private schools of their charitable status and "all other public subsidies and tax privileges", including business rate exemption.
The PM said: "I think it is unbelievable hypocrisy of the Labour Party now to trot out this measure from the 1970s. It's extraordinary that they've excavated this from the crypt of what I thought to be long-buried socialist ideology.
"Let's be clear: this would cost £7 billion of taxpayers' money to educate at public expense all the pupils, all the children who would no longer be educated privately as Jeremy Corbyn was and indeed the offspring of several Labour cabinet ministers."
He added that it was a "pointless attack on the education system".
Asked if he thought he would be prime minister if he had not been privately educated, Mr Johnson replied: "I've been very fortunate in my life in all sorts of ways and I certainly owe a lot to my education."
The motion approved at Labour's conference also called for universities to admit no more than the same proportion of private school students as in the wider population, currently 7 per cent.