A heterosexual couple fighting to enter into a civil partnership -- created for gay couples in Britain -- won a legal challenge on Wednesday to do so, opening the way for others to follow suit.
Charles Keidan, 41, and 37-year-old Rebecca Steinfeld, academics in a long-term relationship but with ideological objections to marriage, argued that restricting civil partnerships to same-sex couples conflicts with equality law.
The partnerships, which confer essentially the same rights and responsibilities as a civil marriage, came into force in England in 2005.
Although same-sex marriage was eventually introduced in 2014, civil partnerships remain available only to gay couples.
Keidan and Steinfeld's claim was rejected by several lower courts, but the Supreme Court agreed that legislation restricting civil partnerships is "incompatible" with their human rights.
The court ruled the pair's entitlement to respect for private and family life guaranteed under the European Convention on Human Rights is contravened by the law.
In a statement outside court the couple described being "elated" by the ruling.
"Today we are a step closer to opening civil partnerships to all, a measure that would be fair, popular and good for families and children across the country," Steinfeld said.
Announcing the court's decision, Judge John Kerr said the government accepted that the legislation treated couples differently and "that the difference cannot be justified."
Lawyers for the couple, who live in west London and have two daughters aged nine months and two, had argued in earlier court hearings that they viewed matrimony as "historically heteronormative and patriarchal".
More than 30,000 people signed a petition backing their case.
Some countries including France and the Netherlands allow heterosexual couples as well as same-sex couples to enter into civil partnerships.