Former adviser to Russian President Vladimir Putin, Anatoly Chubais, has been admitted to a European intensive care ward with a rare immune disorder.
Two sources close to Mr Chubais said the 67-year-old believed he was suffering from Guillain-Barre syndrome, a disease caused by the immune system damaging the peripheral nervous system.
Mr Chubais cannot walk but is able to talk, and his wife Avdotya Smirnova reportedly at his side in hospital.
"He thinks it's a disease," a source told Reuters on condition of anonymity.
"Doctors say they found it in time."
A European intelligence agency was looking into the case but had not disclosed the results yet, that source said.
Some media and opposition activists had speculated Mr Chubais could have been poisoned.
"I don't think it is poisoning," said a second source, who also spoke on condition of anonymity. "He's still in intensive care."
Mr Chubais could not be immediately be reached for comment. It was unclear exactly where in Europe he was hospitalised.
Guillain-Barre is a serious condition that in a minority of cases can cause life-threatening problems such as severe breathing difficulties or blood clots.
Kremlin spokesman Dmitry Peskov said the news of Mr Chubais's illness was sad and wished him a speedy recovery.
His illness was first reported by Russian reporter and political activist Ksenia Sobchak.
Mr Chubais, once former Russian president Boris Yeltsin's chief of staff, was Mr Putin's special representative for ties with international organisations before his resignation.
Mr Chubais quit his post and left the country in March, almost a month after Russia invaded Ukraine.
ABC/Reuters
