The founder of Mumsnet – one of the most influential mums in the UK – has applauded the government for finally waking up to the crippling costs of childcare.

However, Justine Roberts, chief executive officer and co-founder of the online mums’ forum, questioned why the new cash boost is going to families already earning hundreds of thousands of pounds a year.

Ms Roberts, who runs her business from Highgate Road, Kentish Town, is “dumbfounded” that the government will give high-earners tax breaks while cutting child benefit to parents on much smaller incomes.

The mother of four children aged seven to 14, said: “We have been calling for help with the ever-rising cost of childcare.

“While wages remain stagnant, childcare costs continue to rise. Almost a third of a parents’ income is going on childcare. It’s the biggest single cost after the mortgage.

“The fact that they have announced something that means £1billion is going into childcare is good.

“Many, many mothers in particular have either given up work or not accepted a job because of the costs of childcare.

“But there are quite a few concerns. The biggest single concern – or dumbfoundedness – is why they have pitched the tax relief at people who earn so much. A household could earn £300,000 a year and still get a tax break.

“That is inconsistent with other cuts, such as cuts to child benefit and to childcare tax credit.”

The Budget announced that every year working parents earning up to £150,000 each will get up to £1,200 per child under five to help pay for childcare.

This replaces the current scheme in which parents can get tax-free childcare vouchers in lieu of part of their salary – a scheme hindered by the fact that only a small proportion of employers offer the vouchers.

Ms Roberts says childcare costs a minimum of £250 a week in London – and more likely £350 a week.

According to the 2011 census, there were 7,226 women in Camden and 8,548 women in Haringey staying at home to look after the family.