Serious questions are being raised about how the chancellor's 'cash in your annuity' pledge can be realised.

Financial advisers are already reporting inquiries from people wanting to sell on their annuity following the budget promise of a consultation leading to the green light in April 2016.

But industry experts say even the government's own document casts doubt on the feasibility of a secondary market for annuities being created.

They say it makes clear nobody knows how to value an annuity, begging the question how valid expert advice could be given to avoid potential mis-selling. The annuity seller would certainly have to pay for their own advice, while the "buyer's market" would lead to poor value pricing.

John Fox, director of the pension provider Liberty Sipp, said the chancellor "would have to employ the services of Stephen Hawking to come up with a formula for calculating the value of annuities".

Martin Tilley, director of technical services at Dentons Pension Management, said he believed "the likelihood of a real secondary annuity market must be extremely small", adding that IFAs "will not want to provide advice on the sale, as specialist knowledge would be required and the likelihood of a positive recommendation must be slim".

Alan Higham, retirement director for Fidelity Worldwide Investment, said the government' s paper made clear that only that a minority could benefit from swapping their fixed retirement income for a cash sum.

He said: "The consultation document clearly sets out the huge risks involved, not only for the annuitant selling their policy but their dependents too. The consent of the original annuity provider will be required and they would appear to have the unfettered right to say no."

Steven Cameron, regulatory strategy director at Edinburgh-based Aegon UK, said : "Individuals who sell on their annuity for a lump sum are likely to be taxed at their marginal rate. Offering the alternative of 'transferring' the proceeds to a drawdown contract and remaining within a pension wrapper may avoid a tax hit. This would be breaking new ground and needs careful consideration."

MGM Advantage pensions technical director Andrew Tully pointed out that anyone on means-tested benefits with a small pot was set to be excluded, which would "disenfranchise a significant proportion of the people the Government is aiming this policy at".

Just Retirement director Stephen Lowe asked: "What happens to the customer who spends their pension cash after trading in their annuity? Will they still be eligible for state benefits?"

However pensions expert Dr Ros Altmann said there were circumstances in which allowing people to sell their annuity would be sensible. "Those with small pension funds and plenty of other retirement income may welcome the chance to take the cash for urgent expenses or debt repayment. Others may need to provide a pension for a partner which was not included in their annuity. Those with guaranteed annuity rates that only offered single life products will have a chance to cover their partner and those who prefer to leave their pension money invested for a few more years will be able to do so, whereas under the old rules they would have needed huge sums (around £100,000 or more) to be able to use drawdown. Controls on charges or other customer protection might be needed, but at least people will not be stuck for life in an unsuitable product."