In an attempt to head off a Labour rebellion on the issue, which he has been warned would cost Labour support in marginal seats, Mr Brown told MPs at Prime Minister’s Questions that no government “had done more” for childcare and no-one getting support now would lose out.

His official spokesman said afterwards that the government was “always prepared to listen to concerns”, leaving the door open for a switch in a policy row that is taking on the dimensions of last year’s 10p tax revolt on the Labour benches.

The signal of a possible concession came after nine usually loyal former Ministers wrote to Mr Brown warning him that plans to cut childcare is threatening marginal Labour seats in the run-up to the election and more than 40 Labour MPs attacked plans to phase out money-saving vouchers used by many parents.

The former ministers – including Patricia Hewitt, Estelle Morris, Hilary Armstrong, Beverley Hughes and Caroline Flint – say the plans to cut childcare vouchers for more than 340,000 parents are “greatly unfair” and “mark the undoing of one of Labour’s landmark achievements”.

Mr Brown announced he was removing tax relief for employer-based childcare vouchers, arguing that too much of the money was going to the middle classes. He has outlined plans to switch the money to provide 10 hours of free childcare for 250,000 two-year-olds by 2015. But removing vouchers, which are thought to save parents up to £2400 a year on the cost of nurseries, nannies or childminders, would strip “effective and popular childcare support from hard-working parents”, the former ministers said.

The Conservatives have not given a firm pledge to keep the voucher system if they win the election despite its popularity.

More than 70,000 people have signed an online petition against the move and the former ministers believe the Treasury has mistakenly seen the childcare voucher as a middle-class perk.

Their letter said: “Childcare vouchers are an essential support to over 340,000 parents enabling more than 33,000 employers to help their employees, especially women, balance family and work responsibilities.

It added: “Withdrawing them will penalise a significant number of lower-rate taxpayers, reduce the overall amount of funding available for childcare, reduce parental choice and impact negatively on the economy as the UK moves towards recovery.”

The vouchers can be used to offset the cost of childcare from registered providers, saving higher-rate taxpayers £1195 and basic-rate taxpayers £962 a year. Both parents can use the vouchers, potentially saving couples £2390 a year and the government thinks that the scheme is badly targeted by giving too much relief to higher-rate taxpayers.

But the threat to cut the tax perk has angered articulate middle-class parents who are using online sites like Mums-net to register their protest. More than 600,000 parents have used vouchers since tax relief was introduced in 2005. Currently around 30,000 employers offer them and 95% of corporate companies list them as a staff benefit. Thirty per cent of small and medium enterprises are also now on board.

Labour’s backbench critics think the Government figures are out of date and that say the latest surveys suggest 74% of the users of the scheme are basic-rate taxpayers that more than half of the families who claim the childcare vouchers work in the public sector. Downing Street officials said they would look at criticisms carefully in advance of the pre-Budget report, due on December 9.