A heterosexual couple who want to enter into a civil partnership rather than marry have urged a judge to rule that they are being discriminated against.
Rebecca Steinfeld and Charles Keidan, who are both academics and live in Hammersmith, west London, argue that the Government's position on civil partnerships is "incompatible with equality law".
Dr Steinfeld, 34, and Mr Keidan, 39, who have been in a committed relationship since November 2010 and have an eight-month-old baby, want to secure legal recognition of that relationship through a civil partnership.
But the Civil Partnership Act 2004 stipulates that only same-sex couples are eligible.
Their QC Karon Monaghan told Mrs Justice Andrews at a hearing in London on Tuesday: "The claimants submit that the prohibition on access to a civil partnership by opposite-sex couples is directly discriminatory on grounds of sexual orientation since same-sex couples may enter into a civil partnership or a marriage, but opposite-sex couples may only enter into a marriage."
She said the case raised "important matters" for "many couples".
The lawyer told the court in written submissions: "This simple civil mechanism would recognise the claimants as legal partners with attendant legal rights, protections and responsibilities, whilst avoiding the patriarchal rituals and substance associated with marriage."
Without the ability to enter into a civil partnership the couple, who are engaged, would be "forced to enter into marriage against their consciences or remain without the legal protection and privileges that they and their child need, and to which they believe they should be entitled".
At the centre of the case is section 1 of the 2004 Act which requires that civil partners be "two people of the same sex".
The couple are asking the judge to grant a declaration that section 1 is "incompatible" with rights under the Human Rights Act 1998.
More than 30,000 people have signed a petition backing them.
Ms Monaghan said the Education Secretary, who has "responsibility for 'equalities' within Government" - had identified "no good grounds for failing to permit opposite-sex couples to enter civil partnerships".
During the proceedings, expected to last two days, lawyers for the Education Secretary will ask the judge to dismiss the claim.
Dr Steinfeld said in a statement before the hearing: "We are taking this case because the UK Government is barring us, and many thousands of opposite-sex couples like us, from the choice of forming a civil partnership, and we want this to change.
"Personally, we wish to form a civil partnership because that captures the essence of our relationship and values."
"We don't think there is any justification for stopping us or other opposite-sex couples from forming civil partnerships."
Mr Keidan said it was now time for the Government "to demonstrate its commitment to equality by opening up civil partnerships to all couples".
Dan Squires, counsel for the Education Secretary, said in written argument before the court: "In 2013 Parliament decided in extending marriage to same-sex couples not, for the time being, to extend civil partnerships to opposite-sex couples.
"A decision has also been taken by the Government to wait a period of time to see how extending marriage to same-sex couples impacts upon civil partnerships before reaching a final decision on the future of civil partnerships, and has decided that it is not necessary to undertake the costly and complex exercise of extending civil partnerships in the interim where they may be abolished or phased out in a few years."
That decision was "objectively justifiable and plainly proportionate".
He said avoiding "unnecessary costs and disruption pending a final decision on civil partnerships is obviously a legitimate aim".
At present there is no clear indication how civil partnerships are likely to be affected by extending marriage to same-sex couples and "no consensus as to what their future should be".
The hearing continues on Wednesday.