A heterosexual couple who object to the "patriarchal baggage" of marriage are to hear the outcome of their latest battle for the right to enter into a civil partnership.
Rebecca Steinfeld and Charles Keidan want to secure legal recognition of their six-year relationship through that route - but are prevented because the Civil Partnership Act 2004 says that only same-sex couples are eligible.
The academics, who live in Hammersmith, west London, and have a 20-month-old daughter, say that the Government's position is "incompatible with equality law" .
In November, they challenged High Court judge Mrs Justice Andrews's decision to dismiss their judicial review action.
Karon Monaghan QC, told Lady Justice Arden, Lord Justice Beatson and Lord Justice Briggs in the Court of Appeal that the issue was whether the bar on opposite sex couples entering into civil partnerships was incompatible with Article 14 of the European Convention, which relates to discrimination, taken with Article 8, which refers to respect for private and family life.
She said: "They wish very much - and it is of very considerable importance to them - to enter into a legally regulated relationship which does not carry with it patriarchal baggage, which many consider comes with the institution of marriage."
Dan Squires QC for the Secretary of State for Education, who has responsibility for equalities within Government, said that a decision was taken, after two public consultations and debate in Parliament, not at this stage to extend civil partnerships to opposite-sex couples, abolish them or phase them out.
It was decided 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.
He described the judge's decision as "unimpeachable".
Before the ruling on Tuesday, Ms Steinfeld said: "When we started our legal battle for the right to form a civil partnership three years ago, we could never have envisaged the incredible levels of support that would follow.
"Over 72,000 people have signed our petition on Change.org calling on the Government to open civil partnerships to all. We have received support from nearly every major political party.
"It really is remarkable but it just speaks to the fact that opening civil partnerships is popular, fair and would be good for families and children.
"Whatever happens, the campaign has showed why we need mixed-sex civil partnerships. We hope that the Government will take heed and act soon to open civil partnerships to all."
Mr Keidan said that they were "very hopeful".
"We believe we have not only the law but basic fairness on our side. Whatever happens on Tuesday, we will be looking to the Government to open civil partnerships to all couples at the earliest opportunity.
"By doing so, they will be listening to the thousands of people who want to see mixed-sex civil partnerships open to all. This is the simple, fair and popular thing to do."