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David Cameron’s controversial right-to-buy policy was last night branded a “hypocritical bribe and a sham” after it was revealed that even a Tory housing minister questioned whether it could work.

In a letter to Lib Dem MP Tessa Munt in 2013, Tory Housing Minister Kris Hopkins admitted the policy would lead to a “high liability for the public purse”.

Revealing the letter to attack the Tories, Ms Munt said: “This is nothing more than a cynical attempt by an increasingly desperate Tory party to bribe the electorate.

“Even their own housing minister knows it won’t work. It’s deceitful, dishonest and the public will see it for the hypocrisy it is.”

The letter, written in October 2013, stated: “Housing associations are independent, not for profit voluntary bodies and if they are obliged to consistently sell off their stock at less than market value they might find it difficult to borrow which could impact adversely on their repair and maintenance programmes and affect the future provision of affordable housing.”

And Mr Hopkins added: “Any increase to the discount available under the [right to buy] would only be possible through upfront central government subsidy, potentially incurring a high liability for the public purse.”

David Cameron has boasted about allowing tens of thousands of tenants to buy their properties in a Thatcher style council house sell-off.