In a statement faxed from Beijing, where he is receiving medical treatment, the King said he was willing to give up his royal and constitutional immunity and accept a prison term.

"Brother Number 1" Pol Pot died last April

His statement followed growing criticism from human rights groups over the government's failure to arrest two former Khmer Rouge leaders, Khieu Samphan and Nuon Chea, who surrendered late last month.

It came in response to comments, both in Cambodia and elsewhere, that few Cambodian leaders can claim not to have been tainted by associations with the guerilla group and might wish to prevent any trials of its members taking place.

Head of state

Nearly two million are thought to have died in Cambodia's notorious killing fields

King Sihanouk was himself briefly head of state during the Khmer Rouge's period in power in which around 1.7 million people are thought to have died. Fourteen members of his own family were killed.

"Even if the judges of this tribunal do not summon me, I will present myself before this tribunal" the King said, adding he would act as his own lawyer. He said he would accept any decision of the court even he was sent to prison.

Later this month legal experts from the UN are due to decided whether there is sufficient evidence to prosecute Khmer Rouge leaders.

Khmer Rouge guests

Khieu Samphan: "Let bygones be bygones"

Meanwhile Khieu Samphan and Nuon Chena have been allowed to return to the former Khmer Rouge stronghold of Pailin, after touring the country as guests of the government. The area is nominally under government control but is run by ex-Khmer Rouge guerrillas.

The VIP treatment given to the two men has provoked mounting public anger.

Khieu Samphan on December 29: "We have to forget the past"

The Cambodian Information Minister, Khieu Kannarith told the BBC that they allowed the two men to go to Pailin because government lawyers were still discussing procedures for setting up a court.