The Supreme Court unanimously passed a judgement to the advantage of Rebecca Steinfeld, 37, and Charles Keidan, 41, from London.
The court said the “Civil Partnership Act 2004 – which only applies to same-sex couples – is incompatible with the European Convention on Human Rights.”
In a civil partnership, a couple is entitled to the same legal treatment in terms of inheritance, tax, pensions and next-of-kin arrangements as marriage.
The couple, who met in 2010 and have two children, said the “legacy of marriage” which “treated women as property for centuries” was not an option for them.
Ms Steinfeld and Mr Keidan had argued against the notion that only same-sex couples can choose to enter civil partnership or to marry.
The court had earlier overruled their request back in February but their desire has now been granted and ratified by the law.