A senior lawyer has warned that a potential court ruling in favour of opposite-sex civil partnerships could open a legal “can of worms”.

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

Though same-sex marriage was eventually introduced in 2014, civil partnerships are available for same-sex couples only.

This week, the High Court is hearing a case from a straight couple people who claim the treatment is discrimination, and violates their human rights.

Family law solicitor Rebecca Harling, of Thomas Eggar, warned that if the court does rule in favour, this could have far-reaching implications.

She said: “To allow heterosexual couples the option of entering into civil partnerships is unlikely to become law in this jurisdiction mainly for public policy reasons.

“It would mean opening a huge legal ‘can of worms’ which would impact on the whole of the, already over-burdened, family justice system.”

The lawyer added: “This is an interesting case from a family law perspective.

“I believe that Mr Keidan and Ms Steinfeld technically have a point: why should same sex couples have the right to be married or enter into civil partnership whereas if heterosexual couples wish to formalise their legal union they only have the option of marriage?

“It is important to remember that civil partnership was created as an alternative to marriage before same-sex marriage became law in England and Wales.

“The fact that the two options remain is more by accident than design. It is true that the equivalent unions exist in many European countries including France and Holland, but they are civil law jurisdictions and entirely different to England and Wales.”

The government decided against opening up the system to straight people last year after a consultation found little interest, but London-based couple Charles Keidan and Rebecca Steinfeld are demanding their right to a partnership.

The said: “Personally, we wish to form a civil partnership because that captures the essence of our relationship and values.

“Civil partnerships are a modern social institution conferring almost identical legal rights and responsibilities as marriage, but without its historical baggage, gendered provisions and social expectations.

“We don’t think there is any justification for stopping us or other opposite-sex couples from forming civil partnerships.”

The pair contend that Section 1 of the Civil Partnership Act 2004, which restricts civil partnerships to same-sex couples, is incompatible with Article 14 (read with Article 8) of the European Convention on Human Rights, which states that everyone should be treated equally by law, regardless of sex or sexual orientation.

The case has attracted mixed responses from LGBT rights activists.

While veteran campaigner Peter Tatchell is among those to vocally support opening the system up to opposite-sex couples, others are in favour of letting interest in the system decline naturally.

Only a handful of same-sex couples have opted to have civil partnerships since marriages were opened up, putting the system’s long-term future in doubt.