Two sisters have claimed it is unfair that they are blocked from forming a civil partnership.

Civil partnerships were introduced by Tony Blair’s Labour government in 2005, and allowed same-sex couples to register their unions through a separate system, prior to the introduction of same-sex marriage.

This week, an opposite-sex couple’s bid to have a civil partnership was rejected by the High Court – but they’re not the only people who want access to the system.

Sisters Catherine and Ginda Utley have spent most of their lives together, and neither are married. After Catherine fell pregnant in 1993, Ginda stepped up to help raise her daughter.

But despite living as one family unit, the pair – who are heterosexual and not romantically involved – say they have been left without any legal protection.

Catherine Utley has called for the system to be re-purposed to allow for relatives to form non-romantic civil partnerships – which are currently blocked by incest regulations in marriage laws.

She wrote in the Spectator: “The government cannot explain why siblings and other blood relations, who live together in long-term, financially inter-dependent partnerships (frequently also acting as carers to each other, for example, when one is old or ill) are specifically excluded from eligibility for civil partnerships.

“There is nothing in law which says that civil partnerships have to involve a sexual relationship – and nor should there be! They provide rights that are crucial to all long-term cohabiting companions.”

She added: “Why are blood relations barred from them? Why, worse still, are long-term co-habiting blood relations denied every single one of the rights which civil partners, along with married couples (now of both of same and opposite sex), enjoy?

She explained: “My sister and I, both unmarried, and now in our late 50s, have lived together for most of our lives, and in jointly owned property since the 1980s.

“We have also brought up my daughter together since her birth and, having just graduated, she is back living at home.

“But the value of our house, which we bought for £175,000 with a mortgage we have been paying off all our working lives, has risen sharply.

“So much so that, when one of us dies, the bereaved survivor would now have to sell up immediately in order to pay the inheritance tax on the deceased sister’s share.

“The sum the government would extract from us – because it does not approve of relationships based on blood ties – would now be roughly the same as our original joint mortgage all over again.”

The pair say they have a number of prominent supporters – including former cabinet minister Dominic Grieve.

The former Attorney General said: “The basis for creating civil partnerships is the recognition by government of the value of close mutually supportive relationships outside of traditional marriage.

“As such the exclusion of cohabiting blood relations from the right to form one is discriminatory and a serious mistake that needs to be corrected.”