A heterosexual couple who wish to become civil partners rather than marry are taking their fight to the High Court.
New parents Rebecca Steinfeld and Charles Keidan say that they wish to have a civil partnership, rather than a marriage, because they believe it “captures the essence of our relationship and values”.
The couple, who are academics and live in Hammersmith, west London, argue that the Government's position on civil partnerships is "incompatible with equality law".
They launched their case after an official at the Kensington and Chelsea Register Office turned them away because the Civil Partnership Act 2004 stipulates that only same-sex couples are eligible, the Press Association reported.
After starting a petition on Change.org, the couple review the backing of more than 33,000 people.
An online fundraiser has also raised more than £22,000 towards their legal costs.
Steinfeld, 34, said: "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."
She added: "Civil partnerships are a symmetrical, modern social institution conferring almost identical legal rights and responsibilities as marriage, but without its historical baggage, gendered provisions and social expectations.
"We don't think there is any justification for stopping us or other opposite-sex couples from forming civil partnerships."
Keidan, 39, said: "We believe that opening civil partnerships to opposite-sex couples would complete the circle of full relationship equality that began with the hard-won victory for same-sex marriage.
"We campaigned for equal marriage and believe that the significance and symbolism of opening marriage to same-sex couples cannot be overstated."
He said it was now time for the Government "to demonstrate its commitment to equality by opening up civil partnerships to all couples".
Solicitor Louise Whitfield, of law firm Deighton Pierce Glynn, who is representing the couple, said: "This is a clear case of discrimination where the Government has failed to justify its position on excluding opposite-sex couples from all the rights and legal protections that civil partnerships bring.
"This is so important for my clients that they are asking the court to declare that the current situation is incompatible with their human rights, to enable them and many others to access civil partnerships in the future, simply on the basis of equality."
The couple who have an eight-month-old baby, ware set to appear on Tuesday before Mrs Justice Andrews.
Ava Lee, campaign manager of the Equal Civil Partnership campaign, said: "We have received thousands of messages from couples around the country who, for a huge variety of reasons, do not want to get married, but want to celebrate and cement their relationships, and want the same legal protections afforded to married couples.
"Civil partnerships are an institution that already exist, and that would offer exactly that to the ever-increasing number of cohabiting couples in the UK.
"Over 33,000 people have petitioned the minister for women and equalities to ask that she opens up civil partnerships to all couples, yet the Government is still fighting this case."