Chicago (AFP) - A former Black Panther activist who spent a record 43 years in solitary confinement was freed from a US prison after decades of legal battles to prove his innocence.

Albert Woodfox is the last of the "Angola Three" activists to taste freedom in a case which provoked outrage among rights groups.

A federal judge had ordered Woodfox's unconditional release in June in a strongly-worded ruling that barred any further trial on charges of murdering prison guard Brent Miller.

Woodfox twice managed to overturn his conviction for the crime, but Louisiana's attorney general had been determined to pursue a third trial and managed to bar Woodfox's release on appeal.

He won his freedom Friday by pleading "no contest" to two lesser charges in a deal which allowed him to be released on his 69th birthday.

"Although I was looking forward to proving my innocence at a new trial, concerns about my health and my age have caused me to resolve this case now and obtain my release with this no-contest plea to lesser charges," Woodfox said in a statement.

"I hope the events of today will bring closure to many."

The plea is not an admission of guilt but instead a legal maneuver in which he "does not contest that the State would present evidence at a new trial from witnesses who said he committed this crime," his lawyers said.

- Obama pushes reforms -

Lousiana's Attorney General Jeff Landry said in a statement that the plea deal brought "finality" and "closure" to the long drawn out case, adding that the arrangement "is in the best interest of justice."

"Albert Woodfox, by his own plea, stands convicted of the homicide of Brent Miller. In accordance with that plea, he was sentenced to 42 years of incarceration and given credit for time served," Landry said.

President Barack Obama's spokesman Josh Earnest told reporters Friday that the US leader is convinced that solitary confinement should be used "appropriately and sparingly."

Obama, who recently introduced a ban on solitary confinement for juveniles in federal prisons, is pushing to introduce broader reforms to America's overcrowded correctional system -- including a drastic reduction in the practice of solitary confinement in federal prisons.

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"If our ultimate goal of our criminal justice system is to give people a second chance, after they've paid their debt to society, we're basically setting them up to fail if we don't take seriously the long-term negative consequences of prolonged solitary confinement," Earnest said.

"The kind of reforms that the president put forward are the kind of reforms that can only be implemented in the federal prison system," he said.

The Angola Three said they were targeted by prison officials because they spoke out against inhumane treatment and racial segregation at the notorious Louisiana prison built on a former slave plantation.

Woodfox and Herman Wallace, who were sent to Angola for unrelated cases of armed robbery, were convicted of the Rogers murder in 1972.

Wallace was released in 2013 and died shortly thereafter from cancer.

Robert King, the third member of the group, spent 29 years in solitary until his conviction for a separate prison murder was overturned in 2001.

- 'Extreme and cruel punishment' -

Woodfox's attorney George Kendall called his client's long imprisonment "inhumane."

"Albert survived the extreme and cruel punishment of 40 plus years in solitary confinement only because of his extraordinary strength and character," Kendall said in a statement.

"These inhumane practices must stop. We hope the Louisiana Department of Corrections will reform and greatly limit its use of solitary confinement -- as have an increasing number of jurisdictions around the country."

The case of the Angola Three has brought attention to the psychological toll of solitary confinement, which typically means being locked in a tiny cell for 23 hours a day.

Researchers have found that depriving someone of visual stimulation, human interaction, sunlight or physical activity can change their brain structure in a matter of days.

Yet many of the 80,000 people estimated to be in solitary confinement in US prisons have been there for years on end.

"Today should also mark a pivotal new chapter in reforming the use of prolonged solitary confinement in US prisons and jails," said Jasmine Heiss, a campaigner with Amnesty International USA.

"Moving forward, Woodfox's case must serve as a tragic reminder of the cruelty inflicted by the prison system at its most extreme."