By Guy Delauney
BBC News, Phnom Penh
Cambodian Prime Minister Hun Sen has arranged for former Khmer Rouge head of state Khieu Samphan to be airlifted to hospital.
Khieu Samphan was a key figure in the Khmer Rouge leadership
Family members say they believe Khieu Samphan suffered a stroke at his home in Pailin, near the Thai border.
It had been expected that he would be the next senior figure to be charged by the UN-backed Khmer Rouge tribunal.
The tribunal was established to seek justice for the hundreds of thousands of victims of the brutal Maoist regime.
Khieu Samphan's arrest was apparently only days away and it would have completed the tribunal's initial round-up of senior leaders.
Now officials at the special courts must decide whether ill health will force a change to their plans.
WHO WERE THE KHMER ROUGE?
Maoist regime that ruled Cambodia from 1975-1979
Founded and led by Pol Pot, who died in 1998
Abolished religion, schools and currency in a bid to create agrarian utopia
Brutal regime that did not tolerate dissent
More than a million people thought to have died from starvation, overwork or execution
Prime Minister Hun Sen stepped in personally after he heard reports that Khieu Samphan was unwell.
He said he ordered the airlift so that no-one could blame the government if the former Khmer Rouge head of state died.
More than one million Cambodians died when the Khmer Rouge held power in the late 1970s.
But Khieu Samphan has always denied responsibility, despite his senior position.
He has written books claiming that he had no real power and knew nothing about the killings, the latest of which was published this week.
Four senior Khmer Rouge figures are already in custody at the special courts.
Former foreign minister Ieng Sary and his wife, the social affairs minister Ieng Thirith, were arrested on Monday. They are appealing against their detention.