The Conservatives are reluctant to commit to detailed public spending plans ahead of the next election because they consider the Government's economic forecasts to be "junk", David Cameron said today.

The Tory leader told the BBC that he would scrap the national ID card scheme, scale back the NHS database and overhaul the tax credits system if he won a general election, due to be held by May 2010.

Detailed spending plans would have to wait, he said, because the party could not trust growth, spending and borrowing forecasts laid out this week in Alistair Darling's Budget speech, when the Chancellor predicted that national debt would reach almost 80 per cent of GDP.

Mr Darling also forecast that the economy would shrink by 3.5 per cent this year before growth resumes, a prediction that was made to look unrealistic this morning with official figures showing a 1.9 per cent contraction in the first three months of the year.

"The problem with the view ahead – 2011, 2012 – is the Chancellor's forecasts, which we think are probably junk, which means that all his figures for spending are probably junk, [and] his figures for debt," Mr Cameron told the Today programme on Radio 4.

The Conservative leader said that Britain would need a massive culture change to tackle the mountain of public debt racked up under Labour – and the success of a future Conservative would hang on its ability to deal with that "overhang".

Figures in the Treasury “red book” showed that accumulated debt was set to reach £1.4 trillion – 79 per cent of national output – by 2013-14 and that the country would not return to balanced budgets until 2018, forecasts that most experts consider to be optimistic.

Mr Cameron said that if the Tories won the next election they would have to crack down on public spending to make inroads into the debt problem.

Mr Cameron said that, as well as scrapping the ID card scheme, there would be a curb on public sector salaries in what he called the “quangocracy”. The scale of the debt problem would mean a change in the way that government operated, with little scope for ministers to announce new spending projects, he added.

“We are ending the whole approach of big, top-down government,” he said. “The success or failure of a Conservative government, if one is elected, is going to be whether we deal with this enormous problem now facing our country.

“It means that the whole way that government operates is going to have to change. It means no more ministers endlessly announcing new, catchy initiatives with budgets attached to them. It means, actually, ministers being rewarded on the basis of how they can save money rather than spend money.

“There is going to have to be a massive culture change in Britain and it needs a new bunch of politicians to come in and actually deliver it. That is what we are going to have to do.”

Mr Cameron warned that he could not give a commitment to scrap the Government’s new 50p top rate of tax, despite its unpopularity among Tories who argue that it penalises the wealth creators the country needs to lead a recovery.

“It is going to have to form its place in the queue of the tax rises we’d like to get rid of,” he said.

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