Russia's Yukos oil firm, which produces about a fifth of Russia's oil, said production could stop after police confiscated vital computer servers during a raid on its Moscow headquarters today.
"Our central dispatch unit responsible for oil production is in this building. Confiscating servers means damaging the co-ordination of production in our core regions (in Siberia). This means output may stop as soon as today," said company spokesman Mr Alexander Shadrin.
Earlier, Russian special police forces arrived at the Moscow headquarters of the oil company to take over the building, two days after a court froze its assets and gave it five days to pay a $3.4 billion tax bill.
"The OMON (special forces) arrived, they are going to take over our central office," said Mr Shadrin. "They are waiting for the prosecutor. Of course none of the management are working today."
He said company employees would not try to resist the police takeover. "We will wait until Monday and see what is going on."
Yukos has warned it faces bankruptcy because of the immediate tax demand and says that with its accounts frozen, it cannot even sell off assets to keep its operations going.
The $3.4 billion tax demand for 2000, which one Russian news agency reported has been matched by an almost identical one for 2001, follows the arrest of the firm's founder, Mr Mikhail Khodorkovsky.
Many see his arrest as a Kremlin move to stub out the political ambitions of Russia's richest man. He is on trial on charges of fraud and tax evasion.
Russia's government has said it has no plans to re-nationalise Yukos or any of the country's other industrial giants. Many of those firms were bought cheaply off the government in murky privatisation deals during the 1990s.
Last month President Vladimir Putin said he had no interest in seeing the firm - which pumps more oil than Libya - going bust, but the firm says the tax demand will cripple it.