The highest court in the UK has “blurred” the distinction between cohabitation and marriage in its latest ruling, a family law expert has said.

The British Supreme Court ruled yesterday that a man who left his partner 20 years ago is entitled to a 10pc share of the value of their home, while his former girlfriend should receive a “fair” proportion of 90pc, the Daily Telegraph reports.

Prior to the ruling, judges had no power to decide how jointly owned houses and other property should be divided between unmarried partners who separate.

The ruling gives cohabiting couples new rights, similar to those enjoyed by married couples who divorce.

Paul Cobb, the head of family law at Rothera Dowson, said the “landmark” ruling “rewrites the law” on relationships.

“The ruling is widely unexpected,” he said. “This decision brings the rights of unmarried and married couples closer, blurring the traditional distinctions that many thought would require reform by Parliament.”

The case involved Patricia Jones, a hairdresser, and her former partner, Leonard Kernott, an ice cream salesman, who disagreed over how the value of the £245,000 house they shared in Essex should be distributed between them.

The couple split up in 1993 after living together at the house in Thundersley, which they bought jointly, for eight years.

Mr Kernott moved out, leaving Miss Jones to continue paying the mortgage, maintain the house and bring up the couple’s two children, Lauren and Dean, who are both now in their 20s, the court was told.

An earlier ruling at the Appeal Court had concluded that Mr Kernott was entitled to 50 per cent of the value of the property.

But five Supreme Court justices overturned the judgment, ruling instead that he should receive 10 per cent while Miss Jones was entitled to a 90 per cent share of its value.

One of the judges, Lord Wilson, accused Parliament of a “continued failure” to give the courts “limited” powers to redistribute property when a cohabiting relationship breaks down.

Kirstie Law, a partner at Thomson Snell & Passmore, said the ruling represented “a significant change” in the legal landscape for unmarried couples.

“All cohabiting couples should record their intentions with regard to jointly owned property,” she said.

“Couples who do not want a judge imposing his or her own views as to what would be fair given their circumstances will need to regularly review the shares in which they own the property and update any Declaration of Trust if appropriate.”

In Ireland, civil partnership legislation means that a cohabiting couple who have been together for five years or more automatically obtain certain marriage-like rights unless they have reached a private legal arrangement of their own as to the division of property etc in the event of a break-up.