Professor John Appleby, chief economist of the think tank the King's Fund, has said that Conservative proposals to make a wider range of cancer drugs available to patients were flawed and could amount to a "sleight of hand, to say the least".
He told Today presenter John Humphrys that the pledge, which could cost up to £200m a year, was "like making a saving on a tax that hasn't been implemented yet".
And he added that the funding would have to come out of existing budgets.
But the shadow health secretary Andrew Lansley strongly defended the policy. He said that it would be funded by making available money which would be paid by the NHS in increased employers' National Insurance contributions due to be introduced next April, a sum which he estimated at around £200m a year.
Mr Lansley added that there was a "major problem" with cancer drugs in the UK, which he says sees a far lower uptake among patients than other countries. And he called for the government and pharmaceutical companies to collaborate on a different system of pricing for drugs.
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