Cambodia's U.N.-backed war crimes tribunal has ruled a former Khmer Rouge torture chief now on trial was detained "unlawfully" by the military and would be compensated for time served.  The ruling means the only Khmer Rouge official to face justice can no longer be sentenced to life in prison.

Undated photo of Kaing Guek Eav (L), former head of the S-21 detention center and first Khmer Rouge official tried by the UN-backed Cambodian war crimes tribunal and handout (R) taken 30 July 2007 (file photo)

Cambodia's war crimes tribunal says the military violated the rights of Kang Guek Eav in detaining him after his 1999 arrest.

Better known as Comrade Duch, the former head of the notorious Khmer Rouge S-21 detention center was held for eight years before being turned over to the tribunal.

Duch was arrested and is on trial for his role as head of the detention center, charged with war crimes, crimes against humanity, murder and torture.

He was facing up to life in prison if convicted, but will now have all his time in detention subtracted from any sentence he may receive.

The tribunal's legal communications officer, Lars Olsen, says that means at least 10 years taken off, depending on how long the trial lasts.

"And he would also be entitled to additional remedies which the trial chamber in its decision today said they would address when the time for the sentencing would arise," Olsen said.  "That could be typically that he would get extra time off of a sentence because of his unlawful detention."

Olsen says the arrest of Duch was legal, but his detention from 1999 to 2007 was illegal under Cambodian law, which only allows a maximum of three-years provisional detention.

Duch's defense team had asked that he be released from his current detention, but the court rejected that request.

Duch is the only senior former Khmer Rouge official to face justice for crimes committed under its brutal late 1970's rule.  He is also the only one to admit responsibility, although he claims he never personally killed anyone.

Under Duch's supervision, more than 12,000 prisoners there were accused of being enemies of the communist Khmer Rouge and then systematically tortured and executed.

Four more senior former leaders are set to face trial in the next year or two.

Up to two-million Cambodians were worked, starved, and executed by the Khmer Rouge in their attempt to create a Communist utopia.