The move will provide greater security for unmarried couples and their families, the government said.
And it will address the “imbalance” that allows same-sex couples to enter a civil partnership or get married – a choice denied to mixed-sex couples.
The current system was found in June to be in breach of European law.
The Supreme Court unanimously ruled in favor of Rebecca Steinfeld, 37, and Charles Keidan, 41, from London, who launched their own legal bid 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 couple has welcomed the news, but said “legislative action” and a clear timetable was now needed from government.
The Civil Partnership Act 2004 set up a formal framework for same-sex couples, giving them legal and financial protection in case the relationship ended – as in marriage.
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But the law defined them as a “relationship between two people of the same sex”.
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. But they are still not allowed to marry in Northern Ireland.
After these changes in the law, marriage remained the only option for mixed-sex couples.
The government says there are more than 3.3 million unmarried couples in the UK who live together with shared financial responsibilities – nearly half of them with children.
But these households do not have the same legal protections as those who have a civil partnership or marriage.
Mrs. May, who is in Birmingham for the Conservative Party conference, said the move would give all couples the same choices in life.
“This change in the law helps protect the interests of opposite-sex couples who want to commit, want to formalise their relationship but don’t necessarily want to get married,” she said.
The government says unmarried couples in a long-standing relationship often think they have the same rights as married couples.
In fact, they are not entitled to the same tax reliefs and exemptions for spouses and civil partners.
That includes the inheritance tax exemption and the marriage income tax allowance.
Also, if someone’s partner dies, they have no automatic right to inherit their partner’s estate – so they might not be able to afford to stay in the family home.
and then