A legal bid to overturn bans on gay civil marriages and heterosexual civil partnerships is due to be launched at the European Court of Human Rights.
Eight British couples are filing a joint application under the case, which is being brought by campaign group Equal Love.
The four heterosexual and four same-sex couples have all recently been refused licences for civil partnerships and marriages.
They claim they are being discriminated against because of their sexual orientation.
Gay rights campaigner Peter Tatchell, who is co-ordinating the Equal Love campaign, said: "Banning black couples from getting married would provoke uproar. The prohibition on gay marriages should arouse similar outrage.
"The ban on same-sex civil marriages and opposite-sex civil partnerships is a form of legal sexual apartheid - one law for gay couples and another law for heterosexual partners. Two wrongs don't make a right.
"In a democratic society we should all be equal before the law."
More than 40,000 same-sex couples have entered into civil partnerships since they were introduced in 2005.
They mostly afford the same legal rights as married couples, but campaigners believe the arrangement lacks the status enjoyed by marriage.