PHNOM PENH - Former Khmer Rouge prison chief Duch should get 40 years in prison for his role in the hardline communist regime, a prosecutor told Cambodia's UN-backed war crimes court Wednesday.

"We submit... that the sentence to be submitted by this trial chamber should be 40 years in prison," Bill Smith told judges in the prosecution's final arguments.

Duch - whose real name is Kaing Guek Eav - has apologised repeatedly and admitted responsibility for his actions under Pol Pot's brutal communist movement, which killed up to two million Cambodians in the 1970s.

Smith said an appropriate sentence for Duch would be 45 years in prison, although that should be reduced by five years for his general cooperation, limited acceptance of responsibility and expressions of remorse.

"No one should make the mistake that this case is equal to an unqualified guilty plea before an international tribunal," Smith said.

The former maths teacher, 67, faces a maximum life sentence as the tribunal cannot impose the death penalty. A verdict is expected in March next year.

Since proceedings began in February, Duch has begged forgiveness for overseeing the murders of around 15,000 men, women and children at the S-21 or Tuol Sleng prison, a former high school in the capital Phnom Penh.