Samphan, now 76 and the former head of state of the Khmer Rouge's Democratic Kampuchea regime, was widely believed to have been destined to become the fifth person charged by the 56-million-dollar Khmer Rouge tribunal.
News reports said Samphan had become ill Tuesday night. Prime Minister Hun Sen said Wednesday morning in a speech that he was sending a helicopter to Samphan's home in Pailin on the Thai border to bring the elderly man to the capital for treatment.
"If he dies they will blame the government," Hun Sen said.
Sources close to Samphan said he was expected to arrive at the capital's Calmette Hospital soon.
Plans by his family to take him to neighbouring Thailand, where medical care is more advanced, had been dashed when he was refused permission to cross the border, the sources said.
Once a strident critic of Prince Norodom Sihanouk's authoritarian government, Samphan was a founding member of the Khmer Rouge's 1975 to 1979 regime, under which up to 2 million Cambodians perished.
Ieng Sary and his wife Khieu Thirith became the third and fourth people to be charged by the joint UN-Cambodian tribunal on Monday, joining late leader Pol Pot's former deputy Nuon Chea and former S-21 torture centre commandant Kang Keng Iev, alias Duch.