The imprisoned head of Russia's largest oil company has resigned as the firm's chief executive, the latest step in a complex affair that has shaken confidence in Russia's economy.

The chief executive of the Yukos oil company, Mikhail Khodorkovsky, issued a statement from prison saying he is leaving his post as chairman and hoping that the board of directors will be able to carry on successfully.

There were no further details made available about the move more than a week after the 40-year-old businessman was arrested and charged with fraud and tax evasion after a lengthy investigation of his company.

Mr. Khodorkovsky is one of the handful of so-called oligarchs who amassed huge fortunes in questionable privatization deals under former President Boris Yeltsin in the mid-1990s.

On Thursday, prosecutors seized more than half of the shares in Yukos, another move that rattled nerves in business circles and temporarily caused a steep decline in the Russian stock market. Some of the shares were released on Friday.

The case also led to the resignation of President Vladimir Putin's chief of staff, one of the last holdovers in the government from the Yeltsin era. President Putin has tried to ease concerns, meeting with foreign bankers last week. He also appointed a new chief of staff, a trusted aide not allied with Kremlin insiders from the intelligence services, who are seen as supporters of the case against the Yukos company.

The group allegedly launched the investigation into Yukos in July after Mr. Khodorkovsky began funding opposition political parties for a parliamentary election next month.

During the weekend, new chief of staff Dmitry Medvedev expressed concern that the case could have a negative impact on Russia's economy, which has been growing at a steady rate in recent years.

Russia's Finance Minister Alexei Kudrin also tried to calm the markets in an interview Monday in which he says the case against Yukos does not represent a full assault against big business. He added that there would not be any redistribution of property, a comment apparently intended to ease concerns about the seizure of the Yukos shares.

Mr. Kudrin said one case cannot reverse, what he called, the positive changes in the economy and society, and he insisted that the Yukos case is a legal matter for the courts to decide.