Mr Chubais, Russia's most powerful official, made little attempt to play down Mr Yeltsin's condition during a lengthy interview in the Trud (Labour) newspaper, published yesterday.
The aide, whom many believe is running the government, did not suggest Mr Yeltsin should stand down. But he confirmed that a facsimile presidential signature is being used for many documents, although not for presidential decrees.
"It does make things more complicated, both in terms of day-to-day work and in terms of undermining stability in the country as a whole," he said.
Mr Chubais stressed that no key government decisions had been postponed, and that, unlike last year, there has been no damaging infighting at the top. Mr Chubais, considered a pro-Western reformer, attacked the West for seeking to throw a "cordon sanitaire around Russia, from Azerbaijan to the Baltics" through Nato expansion.
His comments will add to the growing sentiment in Russia that the time may be approaching when Mr Yeltsin will be forced to retire. Yesterday, the respected Nezavisimaya Gazeta said the political establishment was looking for a successor.
"Both the opposition and Yeltsin's supporters have clearly demonstrated their alienation from the President", said the newspaper, noting that when Mr Yeltsin was in hospital, a clutch of his ministers and officials, including the Prime Minister, Viktor Chernomyrdin, went on leave. Doubts about Mr Yeltsin's comeback after months of illness deepened two weeks ago when he was admitted to hospital with pneumonia.
The President returned to his residence outside Moscow on Monday, but his aides have fought shy of predicting when he would return to full-time work in the Kremlin.