The move represents a significant, if temporary, shift of power in Russia as the country's mightiest government institutions are now answerable to the prime minister, and are likely to remain so for some time.
The president's spokesman, Sergei Yastrzhembsky, said yesterday that the heads of the power ministries were under instructions to co-ordinate "all questions that require a decision by the head of state" with Mr Chernomyrdin, although Mr Yeltsin would be kept informed of developments.
The order covers the departments of defence, foreign affairs, security, government communications, and intelligence, which usually report directly to Mr Yeltsin and which are widely seen as the roots of power in Russia, after the presidency itself. The Kremlin said that Mr Chernomyrdin will also have the power to convene Russia's recently formed Defence Council if "urgent military issues" arise. However, Mr Yeltsin will remain in charge of the nuclear button.
Last night Mr Chernomyrdin was at pains to stress that Mr Yeltsin stays in charge, and sought to stifle any suggestion that he is intends to assume overall control: "Trust is a mutual thing which has two sides as a minimum. I am convinced that Boris Nikolayevitch can count on our trust. For me, the president will remain president during the operation and during the entire course of medical treatment."
But the president's decision to hand over partial control of some of the key institutions of state came as a surprise, not least because it was sooner than many expected. Only two days before, the president's chief- of-staff, Anatoly Chubais, said Mr Yeltsin would probably relinquish control only "for hours, a day, or two days" following his operation, which is expected later this month.
The decision will be seen as a further vote of confidence by the president in Mr Chernomyrdin at the expense of Alexander Lebed, Russia's security chief and envoy to Chechnya. Although Mr Lebed has urged Mr Yeltsin to pass over the reins of office to Mr Chernomyrdin during his absence, the general also has ambitions to place the power ministries under the oversight of the Security Council, which he heads. He has also been vying with the prime minister for power in what is seen as an open battle over the succession.
Mr Yeltsin has been under pressure to hand over power since his sudden announcement last week that he will be having a heart operation. Under the constitution, executive power passes to the prime minister if the president is incapacitated. But there has been widespread speculation over how he would transfer his responsibilities, and considerable confusion over the laws for doing so. On Monday, the constitutional court ruled that he could make hand-over either by decree or verbally.
Mr Yeltsin has been on vacation in a country hunting retreat since 26 August, but has not announced when he will return to work.