The British ban on marriage between parents-in-law and children-in-law is a breach of human rights, a European court ruled on Tuesday.
The European Court of Human Rights in Strasbourg delivered the verdict on a case brought by a father-in-law and daughter-in-law.
The couple were refused permission to marry by the Superintendent Registrar at Warrington Register Office.
The Department for Constitutional Affairs is considering the judgment.
The man identified in court as B is nearly 60.
His intended bride, his son's former wife identified as L, is more than 20 years younger.
Their relationship developed after B's son left the marital home. B was himself divorced.
The Strasbourg judges said the British ban, although pursuing a legitimate aim of protecting "the integrity of the family", did not prevent such relationships occurring.
The judges said exceptions to the ban were already made in Britain, when it was deemed that "no harm would ensue" from a marriage between parent and child in-laws.
In a previous case, the UK Parliament had even declared that barring the marriage between in-laws "served no useful purpose of public policy"
B and L were awarded nearly £12,000 in costs and expenses in the case.
It means the UK Government must consider changing the law to allow an automatic right for any parents-in-law to marry their children-in-law.