The Tory leader will say that the Government has a duty to make childcare affordable and he favours straightforward tax relief for all working mothers to spend on any form of childcare they choose.

That has been the goal of many parents’ groups, who say that all families should be subsidised for the childcare they use, not only the least well-off who benefit from Labour’s tax credit system. The speech opens up a new front in the battle between the Conservatives and Labour for women’s votes.

Labour introduced childcare tax credits to help to subsidise the cost, but many women complain that they are too restrictive and can be used only for nurseries and child minders. In addition, only low and middle-income earners are eligible.

Mr Cameron will criticise the tax credit system, calling it “top- down, complicated and restricted”. He will say: “Sadly, our childcare costs are now among the highest in Europe. Gordon Brown’s solution is the childcare tax credit.This is symptomatic of a top-down approach. The credit is complicated for parents to claim, it can’t be spent on informal care like that provided by friends and relatives, and its eligibility is restricted.

“In the short term, the existing tax credit system should be extended to cover all forms of childcare, and not just the least well-off families. Our policy review is looking at ways of making the support provided by the childcare tax credit simpler and much more user-friendly.

“Making sure that working parents get the money irrespective of the childcare they use is one simple way of improving the current system.”

The Conservative leader said, however, that eventually he would introduce a simple tax break to benefit all working families. “We must look at the fact that a working man can get tax relief on his mobile phone bill, but a working woman can’t get tax relief for someone who looks after her child. Tax relief on childcare for working parents would end this ano- maly, and our policy review is investigating this,” he will say.

Mr Cameron’s colleagues say that he is personally convinced that tax relief for all is the only solution to the childcare problem. His plans, however, are likely to be challenged by MPs on the Right of the Conservative Party who believe that the Government should instead help mothers who want to give up work to look after their children by restoring the married couple tax allowance.

Mr Cameron has voiced support for the married couple allowance in the past, but his speech today makes it clear that his priority is supporting working parents.

Labour said that a tax break for all working mothers would mean less money for the least well-off. “We currently give an average of £50 a week in tax relief to the 375,000 working mums who need the most support with their childcare,” a Labour source said.

“If we spread that money thinly across all working mums, it would mean an average of just £5 a week per household. So David Cameron is not proposing a tax break — he’s proposing a slap in the face for hard-working families, with lower-income mums losing out the most.”

Mr Cameron’s wife, Samantha, recently returned to work part-time at Smythson, the stationers, after the birth of their third child.

Times Recommends

• No room for Baldrick in overstretched army
• Freecycle booms as middle class feels squeeze
• Marksman gets £5,000 over ‘serial killer’ quip