Gay marriages are legal in 17 US states and Washington DC. Photo: BBC NEWS

The ruling will not apply pending a possible appeal. Same-sex marriages are legal in 17 US states, most of them in the north-east, and Washington DC.

Such unions have been approved either through legislation, court rulings or voter referendums.

'Adding momentum'

The ruling by Judge Arenda Wright Allen in Norfolk followed a legal challenge on same-sex marriage by a gay couple.

The judge agreed with them that the ban infringes on their constitutional rights and their fundamental freedom to marry.

Many expect the case - or one like it - to go all the way to the US Supreme Court, the BBC's Beth McLeod in Washington reports.

Virginia is the first state in the old confederacy - a group of southern states that are traditionally seen as socially conservative - to have its ban overturned.

The decision adds momentum to a growing acceptance of gay marriage in the US, our correspondent adds.

Earlier this week, a federal judge in Kentucky - another southern state - ruled that it must recognise same-sex marriages performed elsewhere in the US.

However, the judge did not rule on the legality of same-sex marriages inside Kentucky, effectively meaning that the current ban remained in place.