Image caption Supporters have asked Louisiana to release Wallace on humanitarian grounds

A judge in the US state of Louisiana has overturned the murder conviction of a terminally-ill prisoner who has spent 41 years in solitary confinement.

Judge Brian Jackson ruled Herman Wallace's 1974 conviction was unconstitutional and ordered him freed.

Wallace, 71, was diagnosed with liver cancer this year and has weeks to live.

He is one of three men convicted of the 1972 murder of a prison guard. Known as the Angola Three, they have maintained their innocence.

One of the three, Robert King, was released in 2001. Wallace and another man, Albert Woodfox, have remained incarcerated, isolated in tiny jail cells and allowed out to shower or exercise one hour a day.

The men, initially imprisoned for robbery, have been the object of a long-running international campaign arguing they were wrongly convicted of the murder because of their association with the militant Black Panther Party.

Baton Rouge, Louisiana, prosecutor Hillar Moore was quoted by the New Orleans Times-Picayune as saying he would appeal against Wallace's release.

'Tragic example'

Louisiana State Penitentiary, where the three men were held, is nicknamed Angola for the plantation that once stood on its site, worked by slaves kidnapped from Africa.

Judge Jackson ruled that Wallace's conviction was unconstitutional because women were barred from serving on his jury, a violation of Wallace's right to a fair trial.

"The record in this case makes clear that Mr Wallace's grand jury was improperly chosen… and that the Louisiana courts, when presented with the opportunity to correct this error, failed to do so," Judge Jackson wrote in his decision.

"The case of Herman Wallace is a tragic example of 'justice' gone wrong in the USA," Tessa Murphy of Amnesty International said in a statement.

"Finally a federal court has acknowledged some of the unfairness surrounding this case. However this sadly comes too late for lasting benefit as he is at death's door with terminal cancer."