(CNN) A heterosexual couple in Britain have lost their court battle for the right to enter into a civil partnership -- an alternative to marriage that is reserved for same-sex couples in the UK.

The Court of Appeal in London rejected a claim by Rebecca Steinfeld and Charles Keidan, who argued they were discriminated against because civil partnerships are not available to people of the opposite sex.

Steinfeld and Keidan oppose traditional marriage because they regard it as a "patriarchal" institution. Speaking outside court, Steinfeld said they were "deeply disappointed" by the ruling.

The 2004 Civil Partnership Act allowed same-sex couples in Britain to enter a legal partnership for the first time. It conferred almost the same legal rights as traditional marriage, except for the right to marry in a religious setting. Same-sex civil partners did not have the right to declare themselves "married" for legal purposes.

Britain legalized same-sex marriage in 2013, but retained civil partnership legislation.

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