The British government has been ordered to bring back an immigrant mother and son deported to Nigeria by Home Secretary Theresa May.

This has been described as a landmark ruling because of its potential impact upon current policy – known as "deport first, appeal later".

Britain must track down and pay for the 45-year-old Nigerian woman (known as BF) and her five-year-old son (known as RA) be returned to the UK by tomorrow (23 April), said Mr Justice Cranston, according to The Independent newspaper.

The judge said the woman's mental health might be harmed by her deportation to Nigeria. She spent years in the UK as an illegal immigrant from 1991 and claimed she would face discrimination as a single mother in Nigeria.

Her asylum claim was rejected in 2010 and a lengthy appeals process eventually ended with her deportation to Nigeria earlier this year. A judicial review of the decision was mounted following her removal.

Judge Cranston, sitting in the Upper Tribunal of the Immigration and Asylum Chamber, said: "In not taking into account the implications of BF's mental health for RA, and the risk of that degenerating in the Nigerian context and the likely consequences of removal, the Secretary of State failed to have regard to RA's best interests as a primary consideration."

The ruling was welcomed by supporters who said concerns about the rights of the woman's child were not taken in to account by the ruling.

Labour parliamentary candidate Ian Mearns, who has represented Gateshead since the 2010 election, said: "This is a massive vindication of the local campaign on behalf of [RA].

"I have been extremely concerned that the Home Office has failed to take into consideration the rights of the child in this case and in others like it."

The Home Office declined to comment on the case.