A heterosexual couple who want the right to enter into a civil partnership are taking their fight to the UK's highest court.
Rebecca Steinfeld, 37, and Charles Keidan, 41, want a legal union through that route but are prevented because the Civil Partnership Act 2004 says only same-sex couples are eligible.
The academics, who live in Hammersmith, west London, suffered defeat at the Court of Appeal in February last year, but were given the go-ahead in August for a Supreme Court hearing.
A panel of five judges, including the court's president Lady Hale, will consider the couple's appeal on Monday.
Ms Steinfeld and Mr Keidan, who have two daughters aged two and eight months, claim the Government's position is "incompatible with equality law".
After being given permission for the hearing, Ms Steinfeld said: "We hope the Supreme Court will deliver a judgment that will finally provide access to civil partnerships for thousands of families across the country."
Mr Keidan said: "The incredible support from many thousands of people who have signed our petition and backing from MPs across the political spectrum has enabled us to come this far.
"What started out as a personal effort to become civil partners has taken on wider significance as we realised that as many as 3.3 million co-habiting couples are affected by the status quo.
"Over the last few years, we've heard the same message: whilst most couples want financial and legal protection for themselves and their families, not all feel comfortable with marriage.
"Civil partnerships offer a legally binding arrangement that is fair, popular and good for families and children."
The Court of Appeal agreed the couple had established a potential violation of Article 14 of the European Convention on Human Rights, which relates to discrimination, taken with Article 8, which refers to respect for private and family life.
But, by a majority of two to one, the judges said the interference was justified by the Government's policy of "wait and evaluate".
They heard the couple have deep-rooted and genuine ideological objections to marriage and wish to enter into a legally regulated relationship which does not carry "patriarchal baggage".
The Government said it was decided, after public consultations and debate in Parliament, not to extend civil partnerships to opposite-sex couples, abolish them or phase them out at that stage.
The aim was to see how extending marriage to same-sex couples impacted on civil partnerships before making a final decision which, if reversed in a few years' time, would be disruptive, unnecessary and extremely expensive.
The couple are expected to make a direct appeal to the Government to support the extension of civil partnerships to all.