Prime Minister David Cameron has said the government will bring forward proposals for a transferable tax allowance for married couples and civil partners.
Once the Marriage (Same Sex Couples) Bill becomes law later this year the policy will also benefit those in same-sex marriages too.
In their 2010 general election manifesto, the Conservatives said recognising marriages and civil partnerships in the tax system would “send an important signal that we value couples and the commitment that people make when they get married”.
But so far the policy has not been introduced.
Mr Cameron has been under pressure from Tory backbenchers to honour the commitment.
Labour opposes the idea and the Lib Dems were given a specific opt out in the coalition agreement which means they do not have to support it.
Deputy Prime Minister and Liberal Democrat Leader Nick Clegg has called the proposals “patronising drivel that belong in the Edwardian age”.
Under plans being considered, wives and husbands who do not work and pay no income tax would be able to transfer part of their annual tax-free allowance to their spouse if their partner earns less than the higher rate of tax, which currently kicks in for people earning £41,451 or more.
In 2010, the Tories said it would make four million married couples and civil partners £150 a year better off.
It is understood that the proposals could be unveiled to Parliament at the time of the Autumn Statement, around the end of November.
Mr Cameron said: “The point is that we are going to be putting in place the marriage tax proposal in law.
“We will be announcing plans for that in this Parliament, quite shortly in fact.”
Last week, Conservative Treasury Minister David Gauke wrote to Tory MPs, saying: “The prime minister has always been clear that it is important to recognise marriage in the tax system, and this firm commitment remains”.
A backbench amendment has been tabled to the Finance Bill by the same-sex marriage opponent and former children’s minister Tim Loughton “to restore a transferable married couple’s tax allowance and send out a clear message that we value marriage and family socially and financially.”
The prime minister said he had not seen Mr Loughton’s proposed change to the Finance Bill, but added: “I don’t think that the amendment is in line with the plans that we have.
“The government is going to do this itself, very shortly, so I think we should let the government get on with it, itself.”
Conservative MP Conor Burns, a strong supporter of equal marriage, believes honouring the commitment could help heal division in the Conservative Party following its split on equal marriage.
He told PinkNews.co.uk last week that implementing the policy could have also bolstered Tory support for the same-sex marriage bill.
“I think it would have been a lot easier to defend the bill if simultaneously we had brought in recognition of marriage in the tax system – and that recognition of marriage in the tax system would have honoured a manifesto commitment and recognised all marriages: gay or straight.”