Labour has unveiled plans for a significant boost to paternity leave if it wins the election - doubling the time fathers can take off to four weeks and adding more than £100 a week to match the minimum wage.

The boost is expected to cost the taxpayer at least £150 million a year if it succeeds in raising take-up by around a quarter - a sum the Opposition says would be more than offset by savings in tax credits from extending free childcare.

Launching what the party dubbed 'Father's Month' as part of a coordinated push of family-friendly policies, leader Ed Miliband contrasted the reform with a Conservative promise of a tax break for married couples.

'The Tories want to spend £700 million on what they call a married couple's allowance but which in fact will go to just one in five families with children,' he said.

'Instead, at the heart of Labour's plan is the belief that Britain succeeds when modern working families succeed.

'That means giving dads, as well as mums, the chance to spend more time at home in those crucial weeks after babies have been born.'

Labour said adopting the reforms, first put forward by the left-leaning IPPR think tank last year, would benefit up to 400,000 families a year.

Under existing rules new fathers qualify for a statutory £138.18 a week, equivalent to £3.45 an hour for a 40-hour week, with employers encouraged to make up the gap in the employee's usual pay.

Only just over half of new fathers (55%) take it up at present,

Bringing the taxpayer-funded contribution up to minimum wage level would increase take-up to around 70%, the IPPR estimates, at a cost to the Treasury of around £150 million in 2015/16.

With the party's spending plans under close scrutiny ahead of the election, Labour said House of Commons figures showed its policy of extending free childcare to three and four-year-olds - paid for by a bank levy - would save 'significantly' more in tax credits than the cost of the extra paternity pay.

'The modern British family needs government to be more flexible in what it does to help,' Mr Miliband said. 'Thanks to the last Labour government, fathers have two weeks' paid paternity leave.

'Millions of families have benefited, with parents saying this has helped them support each other, share caring responsibilities and bond with their children. But the money isn't great - and too many dads don't take up their rights because they feel they have to go back to work so they can provide for their family.'

He pointed to the party's weekend announcement that it would end the 'crazy' situation of underused and mothballed Sure Start children's centres by restoring an obligation on councils to provide childcare via them.

Shadow education secretary Tristram Hunt said using private and charity providers would mean 50,000 new places could be laid on at no cost to the taxpayer - a claim disputed by campaigners.

The Pre-School Learning Alliance welcomed the commitment to restore facilities but said 'it would be short-sighted to suggest that such an initiative would not require additional funding from government'.