The Tories on Wednesday sought to make a major play for the family vote, with an announcement which could save the average family up to £1000 a year.

Announcing plans for a new married couple's tax allowance, William Hague said the strategy would be to focus on families with children aged under 11.

The policy is being seen as a Tory bid to stress the importance of married families and will encourage more parents of young children to stay at home rather than go out to work.

The Conservatives say the plan, which would see a non-working or low-earning spouses transferring their personal tax allowances to their partners, will benefit 3.8 million married couples in the UK.

Setting out the plan, Hague said: "Obviously it doesn't help everyone but it helps those who need it at the time. I think it is a very good way to indicate our support for marriage."

The plan, which would also cover those who stay at home to look after an incapacitated relative, would cost £1 billion a year and would be funded through the £8 billion in spending cuts proposed by Michael Portillo.

The Tories have also set out plans to scrap tax on the widowed mother's allowance and widowed parent's allowance and have revealed proposals to expand the children's tax credit.

Labour has claimed that the Tories' sums still don't add up and have suggested the new "policy on marriage is unravelling within hours". Chief secretary to the Treasury Andrew Smith said Hague's policy was "unfunded, unprincipled and unbelievable". He said: "Far from 3.7 million married couples with children benefiting as Conservatives imply in their selective briefings, the truth is that less than half that number - 1.5 million families - are in the Tories' chosen category. Six million out of seven and a half million families with children will gain nothing."

The Lib Dems have argued that Hague's promises were false. Matthew Taylor, the party's treasury spokesman, said: "The Tories are raining false expectations amongst millions of needy families that their taxes will be cut. But William Hague's Conservatives have in fact failed to explain how they could find the money without cutting funds for schools, hospitals and pensions."