Plans to extend civil partnerships to mixed-sex couples have moved closer to reality after draft legislation cleared the House of Commons.
Tory MP Tim Loughton's Bill would force ministers to make the change within six months of it becoming law.
The government has already promised to extend civil partnerships to all couples in England and Wales.
But it is not backing Mr Loughton's bill, saying the changes needed are "not all straightforward".
Ministers said they wanted to consult on a number of issues including whether couples could choose to convert a civil partnership into a marriage or vice versa.
But despite the lack of government support, the Civil Partnerships Bill secured its third reading in the House of Commons without the need for a vote - and now moves to the House of Lords.
What are civil partnerships?
They were created in 2004 to give same-sex couples - who at the time couldn't marry - similar legal and financial protection to a marriage. They weren't available to mixed-sex couples.
Then, the Marriage (Same Sex Couples) Act 2013 legalised same-sex marriage in England and Wales and the Marriage and Civil Partnership (Scotland) Act 2014 did the same there.
Since then, same-sex couples there have been able to choose between marriage or civil partnerships - except in Northern Ireland, where they are still not able to marry.
But the opposite didn't become true - mixed-sex couples didn't get the right to a civil partnership. That's what's now set to change.
A civil partnership is formed by signing a document.
There is no requirement for a ceremony to take place or to exchange vows - unlike for a marriage - but you can do so if you wish.
In June, the Supreme Court ruled in favour of a heterosexual couple who argued the current UK law was "incompatible" with human rights laws on discrimination and the right to a private and family life.
Mrs May announced her plans to change the law earlier this month.
When will it happen?
Mr Loughton said his Bill had established a "clear timeline" for extending civil partnerships to opposite-sex couples.
He said it was designed to be "helpful" to the government and to "encourage them to get on with delivering their own stated policy".
"Since the Supreme Court's announcement, it has been clear the direction government policy would need to move in but there was growing frustration with the lack of timetable or indeed urgency," he said, appealing to the House of Lords to give it a smooth passage to becoming law.
If the Bill is passed with its current wording, the government would have six months to change the law.
In a statement issued before MPs backed Mr Loughton's bill, Equalities Minister Penny Mordaunt said the government planned to launch a consultation before introducing legislation in the next Parliamentary session, which begins in May.