A transgender student should be allowed to use the bathroom of his choice according to his gender identity, a US judge has ruled.

A Virginia court rejected Gloucester County school board's attempt to dismiss a lawsuit filed by Gavin Grimm, a former student who has since graduated.

The case has been the most prominent in a long and complicated debate over which bathroom transgender students should be allowed to use.

US district judge Arenda Wright Allen's ruling said US federal law backed Grimm's case and said the school's argument was 'resoundingly unpersuasive'. She said policy adopted by the school, which initially allowed him to use the bathroom of his choice before saying he would have to use single-person bathrooms, had 'singled out and stigmatised Mr Grimm'.

Grimm said he felt 'an incredible sense of relief', after the ruling.

'After fighting this policy since I was 15 years old, I finally have a court decision saying that what the Gloucester county school board did to me was wrong and it was against the law,' he said.

The case has followed a long series of appeals after Grimm graduated in June 2017 from Gloucester high school, about an hour east of the state capital of Richmond.

Initially a federal judge ruled in favour of the school, but that decision was overturned by an appeal court citing an Obama administration directive that said students can choose bathrooms that correspond with their gender identity.

However when Donald Trump repealed that directive the US supreme court — the highest in the land — which had initially agreed to hear the case, backed out of the case and sent it back to the district court.

Tuesday's decision will be seen as a major victory for LGBT campaigners.