A challenge against the UK’s bans on gay marriage and straight civil partnerships will be filed at the European Court of Human Rights this week.

The Equal Love campaign, led by gay rights activist Peter Tatchell, seeks to open both institutions up to all couples, regardless of sexual orientation.

The UK legalised civil partnerships for gay couples in 2006, but campaigners say they are not good enough.

Four gay and four straight couples tried to register for ceremonies they were not entitled to in the run-up to Christmas.

Their letters of rejection will be used to form the case. The couples are taking the case to a European court to avoid paying the UK government legal fees if they lose.

Q&A: The Equal Love campaign’s legal case

The court application will be filed on Wednesday morning and Green MP Caroline Lucas will be the keynote speaker at a launch event in Westminster.

Human rights lawyer Professor Robert Wintemute, who is the couples’ legal advisor, said he was confident that they would win.

He said: “Banning same-sex marriage and different-sex civil partnerships violates Articles 8, 12 and 14 of the European Convention on Human Rights.

“It’s discriminatory and obnoxious, like having separate drinking fountains or beaches for different racial groups, even though the water is the same. The only function of the twin bans is to mark lesbian and gay people as socially and legally inferior to heterosexual people.”

He added: “I am confident that we have a good chance of persuading the European Court of Human Rights that the UK’s system of segregating couples into two ‘separate but equal’ legal institutions violates the European Convention.

“I predict that same-sex couples will be granted access to marriage in the UK and that this will be because the UK government will eventually accept that it cannot defend the current discriminatory system.”

Mr Tatchell said: “Outlawing black or Jewish people from getting married would provoke uproar. The prohibition on gay marriages should provoke similar outrage. Arbitrarily excluding heterosexual couples from civil partnerships is equally reprehensible.

“The bans on same-sex civil marriages and opposite-sex civil partnerships are a form of 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.”