Although the seriousness of his condition was unclear, it immediately reignited claims by one of his leading political opponents, General Alexander Lebed, that he is too frail to continue to govern the volatile country. It comes only two months after his multiple coronary by-pass, and follows an 18-month period in which he has suffered at least two heart attacks.
"He must resign because of his health," said General Lebed, who was sacked by Mr Yeltsin in October after a brief stint as his national security adviser during which he made no secret of his own presidential ambitions.
In a stinging assault on his former boss, the retired general described Mr Yeltsin as an "ill old man" who would die if he did not leave office.
The 65-year-old president's ailment was announced on Russian national television news yesterday evening, after he cancelled his appointments for a third day and remained in bed at his residence outside Moscow under the supervision of doctors.
Two weeks ago Mr Yeltsin returned to work at the Kremlin. Although he looked frail, and was several stone lighter after his operation, he declared himself "fighting fit" and began what what was supposed to be a triumphant comeback, proving to millions of sceptical Russians that he was back in charge.
Instead, they have been presented with a depressingly familiar picture - their leader being taken, yet again, to hospital.
Russia endured Mr Yeltsin's lengthy absence from the Kremlin last year without catastrophe, avoiding a political crisis despite the government's failure to pay millions of workers their wages and pensions on time.
The president's ambitious chief of staff, Anatoly Chubais, helped keep the government running amid complaints that he was wielding undue powers.
The Kremlin has repeatedly said that Mr Yeltsin's present ailment is not related to his heart operation. Even so, the fact that he has only managed to survive for a fortnight before heading back to bed is a serious blow to Mr Yeltsin. It comes at a time when Russia faces a number of crises - from continuing unpaid wages and uncollected taxes to a battle to reach an agreement with Nato over Eastern Europe.
Mr Yeltsin will be keenly aware that his last long absence prompted a damaging feud among those eager to wield power - notably Mr Chubais, and General Lebed. The Communist leader, Gennady Zyuganov, the president's rival in last year's election, has also alleged in the past that he is too sick to govern.
In Mr Yeltsin's absence Mr Chubais is likely to take a leading role, aided by the president's daughter, Tatyana. The prime minister, Viktor Chernomyrdin, is also expected to cancel his holiday plans.
The president had been holding regular meetings since his return to the Kremlin, including talks with German Chancellor Helmut Kohl last Saturday, at which the Kremlin leader looked pale and ill at ease. His last meetings were on Monday, when he discussed relations with NATO with top Russian leaders, and held a separate meeting with Mr Chernomyrdin.
Under the constitution, Mr Chernomyrdin would stand in for up to three months before a new election if Mr Yeltsin was unable to govern. He stepped in for a day when Mr Yeltsin had his heart operation. But if the president is bed-ridden for long, both the infighting and the calls for his departure are likely to gather pace and venom.Reuse content