Claire and Martin, who signed a document on the Isle of Man three years ago, will have their union automatically legalised after midnight on Sunday. Rebecca and Charles will be welcomed at a registry office on Monday morning – only five years late.

From Monday, mixed-sex couples in England and Wales can choose to enter into a civil partnership rather than get married. The government estimates there could be as many as 84,000 ceremonies within the first year.

Fears that civil partnership would undermine the institution of marriage have melted away as prominent Conservative MPs have recognised it provides an alternative pathway for millions of cohabitants to protect family life.

Charles Keidan and Rebecca Steinfeld, from west London, initiated the campaign for equal rights for heterosexual couples. They disliked traditional, “patriarchal” marriage and preferred what they say is a more egalitarian commitment. The 2004 Civil Partnership Act, however, permitted only same-sex couples to register their relationship and enjoy similar rights to those who marry.

So in October 2014, Steinfeld and Keidan walked into Chelsea town hall registry office to test the mixed-sex ban and demand full equality. The registrar declined to organise a ceremony, pointing out that they were of different sexes and therefore ineligible.

That refusal prompted a legal challenge in the courts which, after several defeats, resulted in ultimate victory at the supreme court in 2018. In the five years since they embarked on their campaign, Steinfeld and Keidan have had two daughters, Eden, four, and Ariel, two, who will be with them at their ceremony.

Both parents welcome civil partnerships as a simpler arrangement without what they believe is the accumulated baggage of arcane rituals, excessive expenditure and a history of patriarchal dominance.

Steinfeld, 38, a senior policy officer at the charity Maternity Action, said: “We are having the most pared-back event, without frills. 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. This is how you get hitched as a feminist. It’s to remind people about the equality between them and equal division of labour in the household.”

Keidan, 43, who edits a magazine about philanthropy, said: “It’s doing something simply, without a fanfare. It’s a significant moment but at the same time it’s not something that requires opulence and materialism. We will eventually celebrate with a few friends on New Year’s Eve.”

Despite leading the campaign, Keidan and Steinfeld will not be able to enjoy their new status immediately under the Civil Partnerships, Marriages and Deaths (Registration etc) Act 2019 as the first ceremonies will not be held until 31 December.

A Government Equalities Office impact assessment this summer estimated the number of “opposite sex civil partnerships” (OSCPs) it expected over the years 2020 to 2029. Uptake in the first year could be as high as 84,000 or as low as 2,600; the central estimate is for about 27,000 OSCPs to be registered in the first year, with numbers stabilising at about 29,000 a year by 2029.

In other countries that have adopted civil partnerships as an alternative to marriage, they have proved popular. In the Netherlands, which first made them available in 1998, they amounted to 23% of all unions last year.

Tim Loughton, the Conservative former children and families minister, and the Marriage Foundation have both supported mixed-sex civil partnerships on the grounds that they will encourage many of the 3.3 million unmarried couples in England and Wales to formalise their relationships and obtain the legal rights available in marriage.

The first to become civil partners under the new legislation in England and Wales will be those who previously held ceremonies in jurisdictions that already provide them, such as the Netherlands, South Africa, New Zealand and the Isle of Man. Their unions will be automatically recognised from Monday. Anyone who is terminally ill and had been waiting for a civil partnership will also be given permission for an accelerated union.

Among the first to be legally recognised will be Martin Loat, 58, the chair of the Equal Civil Partnership campaign who runs a public relations firm, and his partner, Claire Beale, 53, who is a magazine editor.

Facebook Twitter Pinterest Claire Beale and Martin Loat.

It was when his son’s passport expired just before a school trip and he had to obtain an emergency replacement that Loat was converted. Because he was not married, both parents’ identity documents were required for a new passport to be issued for their son. His partner was abroad at the time, causing bureaucratic chaos.

It persuaded them of the need for an alternative to the institution of marriage. In 2016, they travelled to the Isle of Man and entered into a civil partnership.

“I’m delighted,” he told the Guardian. “It’s been a long battle. Our friends said we were mad going to the Isle of Man. We feel vindicated. We are on the verge of a new social institution and excited about how many people will take it up.

“Claire didn’t like the idea of being a ‘wife’. In the 21st century, a relationship should be able to flourish and blossom and find its own values without anyone having to take a vow.”