The social security secretary, Harriet Harman, conceded the point to the Conservative employment and education spokeswoman Theresa May.

She said Mrs May had made a "very important point" and was "absolutely right" that a difficulty had emerged.

The Tory shadow claimed the government was threatening to abolish the tax break for places in workplace nurseries, introduced by under the last government as a measure designed to help working mothers.

Teresa May: the new Conservative spokeswoman for employment and education

Mrs May said: "The Inland Revenue is now revisiting many of those workplace nursery schemes, removing their tax benefits and causing financial disadvantage to many working women."

Ms Harman replied: "A problem has emerged, we are discussing this with the private sector, particularly with those who have done most to develop nurseries, and of course we are discussing it with the revenue, with the Treasury and I will keep you informed. You have raised a very important point."

'Failed soundbite'

Harriet Harman says lone parents now have more choice

Ms Harman then went on to reject Conservative claims that the New Deal for lone parents is a "very expensive failed soundbite."

Ms Harman said the early figures for the New Deal were "very encouraging" and were providing a better standard of living for lone parents and their families and saving the taxpayer money.

The secretary of state said: "The early results of the New Deal are encouraging. More lone parents are going into work and lone parents and their children are better of on average by £39 a week and the taxpayer is better off by £42 a week."

Despite Ms Harman's claims of success, the Conservative MP for Chelmsford West, Simon Burns, pointed out that of all those invited to attend an interview, designed to get them off benefits and into work, only 25% turned up.

Mr Burns then went on to highlight the fact that 15% of those lone mothers who were taking work under the new deal were falling back into benefits, but Ms Harman brushed aside his remarks, saying the new deal was "well on target with all expectations and costings".