Ministers tried to placate Conservative opponents of gay marriage today by promising that legislation to bring in tax breaks for married couples will be introduced before the 2015 election.

In a letter to Tory MPs, David Gauke, the Treasury minister, said the Government would recognise marriage in the tax system and “legislate for this in this parliament.” The tax break could be worth up to £150 a year and, although no date has been set, it could take effect in April 2015, a month before the general election.

George Osborne, the Chancellor, is unlikely to announce the plan in his spending review tomorrow but it could be included in his autumn statement in November or December.

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Tory traditionalists, many of whom oppose same-sex marriage, have been clamouring for measures to help married couples and stay-at-home mothers. The Coalition Agreement struck in 2010 promised a vote on tax breaks and allowed the Liberal Democrats, who are sceptical about the idea, to abstain in any vote on transferable tax allowances for married couples.

Mr Gauke’s letter has not satisfied all Tory MPs, who are pressing for a firm date by which the tax break will take effect. Tim Loughton, the former Children’s Minister, said: “There is only a certain amount of promises about ‘in due course’ that hard-working families can take. This was a clear Conservative manifesto commitment to deliver a clear and popular Conservative policy that rights an injustice by recognising hard-working families in the tax system.”