More than 3.3 million cohabiting families are being let down by the law, a family law foundation has said.

Resolution, an organisation which promotes good practice in family law, said the number of cohabitees – couples who live together while not married or in a civil partnership – has doubled over the last 20 years.

But the law – or lack of it – for such families remains the same.

Cohabitees have little or no legal protection should they split, despite the myth of “common law marriage”. In the UK, cohabiting couples are treated as individuals for financial purposes, meaning any assets that are solely owned by one partner will not be split upon separation.

Today, Resolution says that the high growth in cohabiting couples is further evidence that the law needs to catch up with modern British Society.

Nigel Shepherd, Resolution Chair, said: “These ONS figures are further proof that more and more couples are choosing to live together and bring up their children without marrying.

“Sadly, some of those relationships will come to an end at some point. This is a feature of our modern society that is here to stay and unfortunately current cohabitation law is failing to provide them with the rights some of them mistakenly think they have.”

Resolution calls for lawmakers to respond and introduce “safety net” legislation which will provide legal protection and fair outcomes at the time of a couple’s separation.

Last year, the organisation released its Manifesto for Family Law, calling for the introduction of some rights for cohabiting couples when they separate.

It was based on research in 2013, which shows that nearly half (47 per cent) of the British public believe in the myth of “common law marriage” – the notion that cohabiting couples have similar legal rights to married people.