The doctor chosen to lead the operation, Renat Akchurin, said last night that the 65-year-old President would have to wait up to two months to minimise the risk. Earlier he had suggested that surgery might be impossible.
The revelations swept like an electric shock through Russia's political establishment, which is well aware that, without the operation, the incapacitated Mr Yeltsin would be a president in little more than name only.
In a measure of the gravity of the situation, Russia's news agencies, the usual conduit for official information, maintained a Soviet-style silence over the president's health. Only cautious mentions were made on television until Mr Akchurin's appearance .
The president's team of surgeons will meet later this week to decide whether to proceed. Mr Yeltsin, who spent a second weekend in hospital, has previously acknowledged that doctors have told him he would have to slow down his life drastically if he does not have surgery.
In effect, he would become a lame duck, capable of working a couple of hours a day at most. His absence would give further momentum to the struggle for power among his inner circle, crushing hopes - felt strongly in the West - that his re-election in July would produce a period of stability.
Previous official attempts to depict Mr Yeltsin's heart bypass operation as routine were made to look silly when senior surgeons made clear that he faces a difficult operation with the risk of complications.
Earlier Mr Akchurin disclosed in a US television interview that the president must have had another heart attack, his third, in late June or early July - between the first and final rounds of the presidential election. It was hushed up to prevent it damaging his re-election chances.
In a separate interview, Mr Akchurin said that the operation might be cancelled because it was too risky, although he said the "most likely [outcome] is that the operation will be postponed. If the risks are high, no one will want to take the chance ... a surgeon does not jump out of a plane without a parachute."
If the operation goes ahead, Mr Yeltsin will hand over his powers to his prime minister, Viktor Chernomyrdin. If the president is incapacitated, or dies, then the Russian constitution states that the premier remains in charge until an election is held, within three months.Reuse content