All couples in England and Wales will be able to choose to have a civil partnership rather than get married, Theresa May has announced.
The move, which follows a Supreme Court ruling, will give mixed-sex couples and their families the option of greater security, the government said.
And it will address the "imbalance" that allows same-sex couples to choose, but not mixed-sex couples.
A couple who campaigned for the right said it was a "major step" forward.
But they said they would only celebrate "when legislation is agreed".
Others have taken to social media to welcome the news and even take the extra step of proposing civil partnerships to their partners.
Catherine Oakley sent a message on WhatsApp to her boyfriend Sam, who she describes as "the embodiment of understatement", asking him to "not marry me".
The government is making civil partnerships for heterosexual couples legal and I just proposed to my partner on whatsapp and he said yes (I’m not crying at work you’re crying at work) @EqualCPs pic.twitter.com/OM7iNsET6Q— Catherine Oakley (@cat_oakley) October 2, 2018
"It's not the most romantic or conventional of proposals," she told the BBC.
"Marriage comes with historical, religious and gendered connotations - this offers a blank canvas.
"It's uncharted territory, so you can make it your own."
The proposed change comes after the Supreme Court, in June, ruled in favour of Rebecca Steinfeld, 37, and Charles Keidan, 41, who wanted to be allowed to have a civil partnership.
The court said that the Civil Partnership Act 2004 was incompatible with the European Convention on Human Rights.
The Scottish government is also now carrying out a consultation on allowing mixed-sex couples to enter into civil partnerships.
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.
Why do mixed-sex couples want this right?
There are a range of reasons people give for not wanting to get married.
High on the list, the feeling among some that marriage as an institution doesn't reflect their equal relationship because of its roots in traditional gender roles.
Ms Steinfeld and Mr Keidan campaigned for four years to get the law changed and more than 130,000 people signed an online petition in support of civil partnerships for everyone.
The couple said the "legacy of marriage... treated women as property for centuries" and was not an option for them.
"We want to raise our children as equal partners and feel that a civil partnership - a modern, symmetrical institution - sets the best example for them," they added.
What protections do you get from a civil partnership?
Beyond the issues of principle, there are many practical reasons for why some unmarried couples might now want to enter into a civil partnership.
As the BBC's legal affairs correspondent Clive Coleman explains, many of the 3.3 million co-habiting couples in the UK believe they possess similar rights and protections to those enjoyed by married couples - but they don't.
It can cause enormous distress when co-habiting partners find out - often on separation or the death of their partner - that they have far fewer inheritance, property and pension rights than they had thought.
Becoming civil partners means couples will get these benefits without having to get married.
The government said there were "a number of legal issues to consider, across pension and family law" and ministers would now consult on the technical detail.
Equalities Minister Penny Mordaunt promised that the change in the law would happen "as swiftly as possible".
How have people reacted?
Mr Keidan said the couple had become "accidental campaigners" on the issue, but told BBC News: "It wasn't just about us.
"There are many, many thousands of couples who would wish to form a civil partnership."
He said he now wanted the law to change and to know when the first mixed-sex civil partnerships would happen.
Ms Steinfeld added: "Charlie and myself can finally formalise our relationship in the way we see fit."
Martin Loat, of the Equal Civil Partnerships campaign, also welcomed it as "another important step forward towards civil partnerships for all".
He added: "Legislation would be fair, popular and promote stable families.
'No longer belonging to a man'
- It's a shame it won't be introduced before April 2019, as that is when I am set to get married and I would much rather be "civilly partnered". Slow process I guess, but good news that consultations are happening. As the legal precedent is now there, it should be legislated for as soon as possible. Salli LaBelle Platt
- I'm so pleased. We have been together since 1979 and have two grown up children. I never wanted to get married, for me it carries too many connotations of "belonging" to a man. Ann Storey
- Richard and I were one of the first couples to sign a Civil Partnership in December 2005. Unexpectedly, Richard died suddenly in 2013. Having the law on my side - especially when it came to dealing with pension rights etc - was important. Stuart Warburton-Smith
Conservative MP Edward Leigh tweeted saying civil partnerships should be extended to include siblings as well.
Why should siblings who've lived together for years have to pay estate duty when one dies? See Lord Lexden's bill earlier this year.— Sir Edward Leigh MP (@EdwardLeighMP) October 2, 2018
He has been campaigning for this change after two sister wrote to him worrying they would lose their family home when the other dies because they would not be able to afford the inheritance tax.