Opposite-sex couples are to be allowed to enter into civil partnerships, according to Prime Minister Theresa May.

Until now, only same-sex couples have been allowed to contract civil partnerships, with marriage the only option for heterosexual couples who wish to enter into a formal commitment. However, pressure for a change in the law from activists increased with the Supreme Court victory in June of Rebecca Steinfeld, 37, and Charles Keidan, 41, from London, who argued that the current legislation was discriminatory.

The prime minister has told the Evening Standard that gay and straight people will have 'the same choices in life'.

She 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.'

Martin Loat, chair of the Equal Civil Partnerships campaign group, hailed it as an 'important step forward', saying: 'Legislation would be fair, popular and promote stable families.'

An estimated 3.3 million couples, half of them with children, live together without being married. Many believe they have the rights of married people without formalising their relationship, but this is not the case.

Campaigners argue that those with objections to the institution of marriage should be free to enter a contract that safeguards property and inheritance rights without marrying.

Equalities minister Penny Mordaunt told the Standard the latest measure would be 'an important step forward for equality' and promised legislation will be brought before MPs 'as swiftly as possible'.

However, a spokesman for Christian social policy charity CARE criticised the move. James Mildred said: 'Coming off the back of the Government's rush to implement no fault divorce, there is a real narrative developing that this is a Government unwilling to back marriage.

'Marriage is the gold standard of commitment. It should be made more fiscally accessible given the huge benefits it brings to adult well-being and child development.

'We have historically low marriage rates in this country and family breakdown is estimated to cost a staggering £51billion a year. It is entirely possible that today's decision will only make things worse, not better.'