Albert Woodfox
Albert Woodfox has been held in solitary confinement since 1972Amnesty

A US prisoner who has spent the last four decades in solitary confinement in a Louisiana jail has been ordered for release by a federal court.

Albert Woodfox, the last of a group of prisoners known as the Angola Three, has been held in solitary confinement since April 1972 following a prison riot at Louisiana State Penitentiary which left one guard dead.

Woodfox was tried twice for the murder of guard Brent Miller, but had both convictions overturned. US District Judge James Brady has now ordered the release of Woodfox after taking the unusual step of blocking prosecutors from trying him for a third time.

Bradley said in his ruling there were "exceptional circumstances" in Woodfox's case which led him to bar prosecutors from seeking a third trial. These include Woodfox's age and poor health, lack of available witnesses and the "prejudice" done onto Woodfox as a result of his 40 years in solitary confinement.

Prosecutors are said to be planning an appeal "to make sure this murderer stays in prison and remains fully accountable for his actions".

Woodfox, who is the longest-serving solitary confinement prisoner, has always denied being responsible for Brent's death.

Woodfox was part of a group of inmates known as the Angola Three, so called as the maximum security state prison where they were held in solitary lies next to a former slave plantation called Angola.

The other two members of the Angola Three were Herman Wallace and Robert King. Wallace and Woodfox, both serving unrelated armed robbery sentences, said they were both treated harshly in the prison as they were involved in the black military rights group the Black Panthers.

Wallace, who was convicted in Brent's murder along with Woodfox, was freed in October 2013 on compassionate grounds after being diagnosed with terminal liver cancer following a long battle with the Louisiana authorities. He died from the disease just days after his release.

King, who was convicted in the death of a fellow inmate in 1973, was released in 2001 after his sentence was overturned following 29 years in solitary.

Jasmine Heiss, a campaigner with Amnesty International USA, described Woodfox's ruling as "a momentous step toward justice".

"The only humane action that the Louisiana authorities can take now is to ensure his immediate release," she added.