David Cameron was put under pressure to explain how the Conservatives would fund a string of tax cuts as the party announced plans to spend £200 million on getting the unemployed back to work.

The Tory leader promised he would set out his plans on expenditure and taxation before the next election, but he emphasised that the specific details would not be decided until much closer to that date.

Mr Cameron has been criticised for promising a series of tax-busting measures — including a reduction in the rate of corporation tax and a scrapping of vehicle excise duty — without making clear how he would fund the proposals.

Yesterday the Conservatives said that they planned to offer employers a £2,000 bonus — on top of money they already receive — for taking on and training apprentices as builders, plumbers and electricians.

With a goal of training 100,000 apprentices in total, an incoming Conservative government would need to find another £200million at a time when the public finances are under severe strain.

David Willetts, the Shadow Skills Secretary, said that the scheme would be fully funded by “refocusing” money from the department's budget. He declined to give details, but said that they would be outlined when he and Mr Cameron presented the initiative on Wednesday. “We will for the first time be covering the full cost of an apprenticeship,” Mr Willetts told The Times. “We're saying to employers we'll lower the hassle and give you a bigger incentive.”

Mr Willetts said that many social problems stemmed from “underperforming white working-class men” who were disengaged from society. “We want to provide more opportunities, particularly to young men, to get them off the streets and on their way in life.”

Far from making promises that could not be financed, he said that he had often “disappointed” Tory colleagues by refusing to offer unfunded tax cuts or spending increases. He said that he was not prepared to write the budget for 2010 now. “I don't think it would be responsible to rewrite your budget every month according to changing circumstances,” he told The Andrew Marr Show on BBC One.

“We have always been very clear about where the money comes from, how we pay for tax cuts. At the next election we will be absolutely clear.” Asked whether he would cut spending, increase borrowing or raise taxes to cope with the difficulties his government would inherit, Mr Cameron said: “ I think it is completely unrealistic to ask, possibly two years before a general election, when we're only a few months into this financial year.”

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