They are currently prevented from having a legal union through a civil partnership because the Civil Partnership Act 2004 says only same-sex couples are eligible.
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, according to the government.
These couples do not have the same legal protections as those who have a civil partnership or marriage. In a civil partnership, a couple is entitled to the same legal treatment in terms of inheritance, tax and pensions as in marriage.
The Prime Minister said: “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.
“As home secretary, I was proud to sponsor the legislation that created equal marriage.
“Now, by extending civil partnerships, we are making sure that all couples, be they same-sex or opposite-sex, are given the same choices in life.”
The announcement comes after a heterosexual couple fighting for the right to obtain one won a ruling at the Supreme Court in June.
The judges granted a declaration that the 2004 Act was “incompatible” with human rights laws on discrimination and right to a private and family life.
Gay couples were denied access to marriage until 2013, when an amendment to the 2004 Civil Partnership Act allowed same-sex couples to marry in England, Scotland and Wales.
A civil partnership gives a couple similar legal rights in terms of parental responsibilities, property, tax, inheritance and next-of-kin arrangements, to a married couple.
Sarah Jane Boon, partner at law firm Charles Russell Speechlys, said: “Theresa May’s announcement is a welcome, if overdue, development given the ongoing debate about the need to recognise the rights of unmarried couples who cohabit for significant periods, and whose relationships are akin to those of married couples.
“However, it is far from a complete solution and today’s announcement should not divert attention and resources away from the core issue. The stark reality is that many cohabiting couples simply do not wish to enter into a legal union and would not do so, regardless of the label, whether marriage or civil partnership.”