A campaign group is launching a legal challenge in the European Court of Human Rights to extend the rights of gay couples to full marriage.
Equal Love is also arguing that heterosexual couples should be entitled to form civil partnerships.
More than 40,000 same sex couples have had their relationships recognised in law since civil partnerships were introduced in 2005.
The Equal Love campaign will see eight couples file a court case.
The couples, four heterosexual and four same-sex, have all recently tried to marry or form a legally recognised civil partnership and been refused for various reasons.
Civil partnerships mostly give same-sex couples the same legal rights as married couples, but some campaigners believe the arrangement lacks the status enjoyed by marriage.
The four same-sex couples will file a case with the European Court of Human Rights as part of the Equal Love campaign, arguing that they are being discriminated against because of their sexual orientation.
At the same time the four heterosexual couples, who have attempted to form civil partnerships and been turned down, will do the same.
Each group wants the rights afforded to the other and insists the government has an obligation to consistency.
Ian Goggin, who is in one of the heterosexual couples involved in the case, told BBC Radio 4's Today programme that there were two main reasons why he and his partner Kristin Skarsholt preferred civil partnership to a traditional marriage.
He said: "One is that we can't buy into marriage whilst it excludes our homosexual friends. We can't condone that by entering into a marriage.
"And secondly, we feel a civil partnership better reflects our relationship by allowing us to be legally recognised as partners - and also meaning that we can not have to buy into the culture and language of marriage."
Mr Goggin added that even if the law was changed so that homosexual couples were allowed to marry in a traditional way, the pair would still want a civil partnership.
He said: "We still don't identify with the culture and language of marriage. This isn't just a gay rights campaign, it is a human rights campaign."
The case could take three years, but campaigners are hoping government policy may change before that.
Gay rights campaigner Peter Tatchell told BBC Radio 5 Live that the situation was currently unfair.
"When the law says a gay person is banned from marriage, that sends a signal. It sends a signal that we are regarded as socially and legally inferior."
He added: "No-one grows up dreaming of being a civil partner. People grow up dreaming of falling in love and one day getting married. That dream is just as real for gay people as it is straight people."
The campaign has been backed by Terry George, who was one of the first people to enter into a civil partnership five years ago.
He said gay couples should have the same rights as heterosexuals: "I can understand where the people who are campaigning are coming from because I think it should be equal for all.
"It seems that public attitudes have changed, but people in church and religion still find it repulsive that two men or two women would want to love each other and have a civil partnership or a marriage."
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