Civil partnerships were introduced for same-sex couples in 2004 and represented the only way for many people to formalise their relationship until the introduction of same-sex marriage in 2014.

Concerns that mixed-sex civil partnerships would undermine the institution of marriage have gradually faded away as it has increasingly been seen as offering a route for cohabitants to secure their rights. Under the terms of a civil partnership, a couple is entitled to the same legal treatment in terms of tax, pensions, inheritance and next-of-kin arrangements as if they were married.

The campaign for heterosexual civil partnerships was spearheaded by Londoners Charles Keidan and Rebecca Steinfeld, who took their case to the Supreme Court in 2018. The pair said they did not agree with the “legacy of marriage” that had “treated women as property for centuries”.

Steinfeld, who works for the charity Maternity Action, told The Guardian newspaper: “I hope that civil partnerships will be very popular. There are a lot of people like us who want the legal status and financial protection but don’t feel that marriage is the right fit for them.”

While people can give their notice of intent to form an opposite sex civil partnership from Monday 2 December, the first ceremonies will not take place until 31 December.

The government predicts that uptake could be as high as 84,000 in the first year, with numbers stabilising at around 30,000 a year by 2029.

Civil partnerships have already proved popular as an alternative to marriage in other countries. In the Netherlands, where they have been an option since 1998, they amounted to almost a quarter of all unions in 2018.

There are an estimated 3.3 million unmarried couples currently living in England and Wales who could take advantage of the new arrangements.