David Cameron’s pledge to offer a right to buy to all housing-association tenants has been questioned by housing lawyers and a former civil service chief.

The controversial policy, a core plank of the Conservative manifesto, sparked uproar as social landlords warned that selling off their housing stock at a discount, including older properties built without any government funding, would make it harder to build replacement affordable housing.

Lord Kerslake, who will attack the flagship housing policy for England in his maiden House of Lords speech next week, told The Observer that giving housing association tenants the right to buy their homes will do nothing to solve the housing crisis.

Lord Kerslake said: “I will raise my serious concerns about the policy in its current form.

“I think it’s wrong in principle and wrong in practice, and it won’t help tackle the urgent need to build more housing and more affordable housing in this country, particularly in London.”

A briefing note accompanying the Queen’s speech last week stated that a forthcoming Housing Bill would “enable the extension of right- to-buy levels of discount to housing association tenants”.

Lawyers say that the wording of the pledge would allow the UK government to widen access to home ownership by extending the “right to acquire” rather than introducing a new right to buy.

The right to acquire currently offers minimal discounts – limited to £16,000 – to small numbers of housing association tenants who want to buy their own home. If the discount were increased to match the maximum available under the existing right-to-buy scheme, it would be £103,900 in London and £77,900 across the rest of England.

But extending the right to acquire would limit the sell-off. It would allow housing associations to protect properties that are specially adapted or designed for sheltered accommodation, and they could make a strong legal case for exempting all properties built before 1997, many of which were never funded by government.

One social housing specialist told The Independent on Sunday that the policy was a “fudge”. “When the original policy was released, it raised a few eyebrows. Most of the Conservatives thought they probably wouldn’t get it through Parliament. They are going to have to go through with it, but the devil is very much going to be in the detail,” he said.

Catherine Hand, a partner at Trowers & Hamlins law firm, said it appeared that the government had “not fully decided how it’s going to do this, and wants to give itself some space. The idea that they can make somebody sell those assets is a much greater hurdle and I think there could be real disquiet among some MPs. There are bigger, better things for the Government to be thinking about.”

Mark Upton, a policy analyst and former civil servant, also noted that the briefing on the Housing Bill used “a strange form of words”.

“I do wonder if this is what they were proposing all along. There is always a different kind of language in politics as opposed to policy. It’s a simpler soundbite to say we’re extending the right to buy than to say we’re making reforms to the existing right to acquire,” he said.

More details of the scheme are set to be announced by the Department for Communities and Local Government in the coming weeks.

A department spokesman said: “Anyone who works hard and wants to get on the property ladder should have the opportunity to do so. That’s why the Housing Bill will ensure that social tenants wanting to buy their home would have the opportunity to do so at the same levels of discount – regardless of whether they live in a council or housing association property.”