Thousands of heterosexual couples in England and Wales are expected to enter into civil partnerships later today.
The unions will take place after a long legal battle against the law which had only permitted same-sex couples to become civil partners.
The partnerships offer almost identical rights as marriage, including property, inheritance and tax entitlements.
The government estimates 84,000 mixed-sex couples could become civil partners next year.
The changes come after heterosexual couple Rebecca Steinfeld and Charles Keidan won their legal bid at the Supreme Court in 2018 for the right to have a civil partnership instead of a marriage.
As a result, rules were changed to extend civil partnerships – available to same-sex couples since 2005 – to everyone.
Five years after being refused permission to give notice of a heterosexual civil partnership, Rebecca Steinfeld and Charles Keidan will finally become civil partners today.
Their conscientious objection to marriage and what they saw as its patriarchal associations led to a lengthy legal battle culminating in a unanimous Supreme Court ruling last year that the law was discriminatory and breached their right to a family and private life.
The government changed the law, opening such a union to the majority of the UK’s 3.3 million co-habiting heterosexual couples.
Many believe they are already protected by so-called “common law marriages”, but these do not exist.
As a result, they do not enjoy the same property, inheritance and tax entitlements as married couples and civil partners.
The government estimates as many as 84,000 mixed sex couples could become civil partners this year, giving them greater rights and protections within their relationships, without having to get married.
Another couple, Julie Thorpe, 61, and Keith Lomax, 70, said they were looking forward to being among the first mixed-sex people to officially enter a civil partnership – but it would not change their relationship “one jot”.
The couple from near Hebden Bridge, West Yorkshire, have been living together for most of their 37-relationship and have three children.
They will have a civil partnership ceremony at a register office in Halifax.
Ms Thorpe said: “It won’t change our relationship one jot. It will not make any difference to how we behave towards each other when we get up the next day.
“We have had a very successful relationship for 37 years and a bit of paper is not going to make any difference to that whatsoever. It does give us some legal protection within that relationship.”
Mr Lomax, a human rights lawyer, added: “It is a mutual celebration of all of those and also of the people who actually brought the case to court and changed the law in the first place, because that was a very brave and bold thing to do at considerable financial risk.”