Unmarried couples who split have won equal rights to a share in their home - but only if they put it in joint names, the House of Lords ruled this morning.

In a landmark judgment, affecting up to two million cohabiting couples, a majority of the law lords, Britain’s highest court, ruled that where a couple own a house in joint names there should be a presumption that they own it equally.

Their ground-breaking decision for the first time gives some legal credence to the widely-held myth that a “common-law” marriage does carry property rights.

It means that when a relationship splits, the former cohabitants are a step closer to “divorce-style” rights with each gaining half the value of the home when sold.

But as with divorcing couples, courts will take account of factors such as how much each party put into the relationship and work out their share accordingly.

And the law lords gave a stark warning that unmarried couples could face an expensive battle when they split up - with legal costs swallowing up any assets available, as in today’s case.

If the property remains in one person’s sole name, the rights do not apply - and that person will still have the burden of going to court to argue for their share.

The ruling involved a couple who had lived “as man and wife for 20 years” and is the first case on how unmarried couples should divide their assets to come before the House of Lords.

But it brought no joy to Barry Stack, 51, who had challenged the appeal judges’ decision that his former partner Dehra Dowden, 49, should keep 65 per cent of the proceeds from the sale of their £770,000 home in North London.

The law lords said that in this case, Miss Dowden should keep the 65 per cent because it was an “unusual” case that fell outside the 50-50 equality principle in that she had made the greater financial contribution.

The couple, from Willesden Green, who had four children together, had lived together for 20 years.

They have run up large legal bills to determine the respective shares of their London suburban home because they failed to draw up a “declaration of trust” which would have made the position clear when they bought it.

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