In what is being seen as further evidence of the shift in power in Russia, Mr Yeltsin's deputy chief of staff, Oleg Sysuyev, said that during the rest of his term - which expires in the year 2000 - the president would concentrate on revising the constitution so as to ensure a stable transition to his eventual successor.

Andrew Harding: It is an "open secret" that Mr Yeltsin is too sick to run the country

Moscow correspondent, Andrew Harding, says this means the Prime Minister, Yevgeny Primakov, is taking over most of the president's responsibilities.

Mr Primakov would be in charge of the economy, while Mr Yeltsin would retain a largely ceremonial role.

In practice that has already happened, but our correspondent says Mr Yeltsin is now prepared to go one step further and amend the constitution accordingly.

Rare appearances

Earlier this week, Mr Yeltsin was admitted to a sanatorium near Moscow.

A Kremlin spokesman announced the "exhausted" president had gone to the Barvikha sanatorium to recuperate, a day after doctors ordered him to cancel a trip to the European Union summit in Austria because of "unstable blood pressure" and extreme fatigue.

Boris Yeltsin stumbles in Central Asia

He was forced to cut short a trip to Central Asia earlier this month because of what the Kremlin called a respiratory infection.

Mr Yeltsin has rarely been seen since the start of Russia's financial crash in late August.

Yeltsin era 'over'

Russian opposition forces have this autumn intensified their calls for Mr Yeltsin to step down.

"The Yeltsin era is over. We should speak of it less and less," said Communist Party leader Gennady Zyuganov this week.

"We cannot look at him without being shocked. The best thing for Russia would be that Mr Yeltsin stands down, but he does not have the will to do that."