David Wilson: One of the three murdered backpackers

Judge Thong Ol told a stunned court that the decision to acquit the defendant had been made on the basis of a law that indemnifies Khmer Rouge members who turned themselves in of all crimes committed while they were a part of the organisation.

Welcoming the decision, Chhouk Rin, who was made a colonel in the Cambodian army after his defection, smiled and bowed to court officials.

"I have a belief in our courts as really independent and our country is a lawful state," he told reporters after the trial.

Anger at verdict

Relatives of the three dead men, who had travelled to the Cambodian capital, Phnom Penh, for the trial, reacted with shock and anger to the verdict.

Jean-Claude Braquet, whose son was one of the three dead tourists, stood up and shouted at the judge before storming out of the building.

The three backpackers - one French, one Australian and one Briton - were seized in 1994 whilst travelling on a train bound for the coastal resort of Sihanoukville.

They were later found murdered after attempts had been made to extort a ransom.

Trial 'travesty'

At the opening of the trial earlier on Tuesday, Chhouk Rin told the court: "I did not participate because at that time I was very sick and under treatment.

Millions died during the Khmer Rouge's brutal rule

"If I was there, I would have had to be carried."

Another Khmer Rouge guerrilla, Nuon Paet, has already been sentenced to life imprisonment for his role in the murders

William Wilson, lawyer for the family of murdered Australian David Wilson, described the decision as a "travesty of justice", saying it was grossly unfair for Chhouk Rin to be let off on what amounted to a technicality.

He said the verdict had obvious implications for the ability of the Cambodian judiciary to conduct an independent trial of former Khmer Rouge leaders.

Earlier this month, the United Nations and the Cambodian government agreed a provisional deal on genocide trials for surviving Khmer Rouge leaders.

The Khmer Rouge regime, led by the late Pol Pot, is believed to have been responsible for as many as two million deaths during its brutal rule in the 1970s.