Jeremy Corbyn has pledged to vote with the Tories if they introduce new rights for transgender people.
The Labour leader said trans discrimination has gone on too long and called on Theresa May to act following her admission during the general election that "changes do need to be made".
Reforms allowing people to legally chose their gender and end intrusive medical checks must be made, he told a Pink News reception in Parliament.
The Prime Minister's deal with the socially conservative DUP means she would struggle to push through reforms to the Gender Recognition Act.
But Mr Corbyn said his MPs would back Mrs May in the voting lobbies to help bring in the changes.
He said: "Fifty years ago, homosexuality was decriminalised. Today, the struggle goes on for LGBT rights for all. In particular, we must advance trans rights. Discrimination has gone on too long.
"The Gender Recognition Act does not allow trans people to self-identify their gender and forces them to undergo invasive medical tests. This is wrong.
"Labour recognised this in our manifesto, pledging to update the Act. Theresa May told Pink News that 'changes need to be made' but failed to include anything on this in the Conservative Manifesto.
"So, I say to her today, Labour will help you keep your promise. Bring forward a Bill to update the Act and improve trans rights and Labour will back you. You can give your MPs and the DUP a free vote and Labour will make it law."
Commons Speaker John Bercow, who hosted the event, said gay marriage will only be really equal when it can also happen in church.
He said: "We are marking 50 years since the partial decriminalisation of homosexuality.
"There's always a balance to be struck - we don't want to behave as though it's all over, that everything has been done. Nothing remains - because that isn't true.
"I still feel that we can only really have equal marriage when you can bloody well get married in a church if you want to do so."