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David Cameron will have a Mother's Day card delivered to his door by campaigners for new mums whose benefits are about to be capped.
Labour has accused the Government of imposing a "mummy tax" and said the welfare reforms are part of a series of austerity measures which unfairly target mothers.
Shadow minister for women Yvette Cooper MP told Sky News: "It's like David Cameron and George Osborne have a blindspot about women because they're paying three times more than men in tax and benefit and pay and pension changes.
"That is so unfair when women earn less and own less than men.
"It shows that the Prime Minister and the Chancellor just don't get it and it's outrageous that new mums are hurt hardest."
The criticism came as the Archbishop of Canterbury was among 43 bishops who have written an open letter condemning the Government's plans to change the benefits system, saying it will have a "deeply disproportionate" effect on children.
The Most Rev Justin Welby has warned that "children and families will pay the price" if plans to change the system go ahead in their current form.
Around 340,000 women claim either statutory maternity pay or maternity allowance every year.
Until now their benefits have gone up in line with inflation, which currently stands at 2.7%, according to the Consumer Price Index.
But from next month new mothers' benefits will go up by just 1% every year as part of a three-year cap on welfare increases.
So by 2015 critics have calculated the benefits will be effectively cut by £180 because they will not increase by as much as the cost of living will.
Conservative MP Amber Rudd said: "The fact is there are so many good things we are doing to try to help mothers.
"What mothers really want is welfare that works, improved education and jobs.
"That's what they talk to me about on the doorstep and I feel this Government is doing a lot on that front.
"And it's rank hypocrisy of Labour to accuse us on this front when they have made no suggestions about how to reduce the deficit."
And Schools Minister, Liberal Democrat David Laws MP, also defended the welfare reforms and said the Coalition had tried to help those on lower incomes.
He told Sky's Dermot Murnaghan: "We've had a public sector pay freeze. We've also had a 1% cap in the future on public sector pay. So we've have had to take difficult decisions not just for some of those on lower incomes but for everybody in society.
"And actually we've tried to help some of those on lower incomes by raising the tax free personal allowance and also exempting some of the lowest paid public sector workers from the effects of the pay freeze."
Single mum-to-be Helen Mockridge has one clear suggestion for a better way to reduce the deficit.
"Taxing really rich people, obviously, that's where the money should come from," she said.
"For me it's a real no-brainer and it makes me really angry that certain parts of society are very, very wealthy and the gap between rich and poor is getting bigger.
"That's where the money should be coming from, not from single mothers or the disabled or any other vulnerable group."
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