Mr Khodorkovsky said he would concentrate on charitable work

He said the detention of the company's chief executive, Mikhail Khodorkovsky, was neither evidence of authoritarianism nor a sign that Moscow was considering renationalisation of privatised industries.

Speaking to Italian journalists ahead of a state visit to Italy, Mr Putin said the Yukos case was about law enforcement.

"There will be no de-privatisation or revision of privatisation results. But one and all should learn the skill of living by the law and observing the law of the country," Mr Putin said.

'Political ambition'

Shares in Yukos rose by 12% following the resignation of Mr Khodorkovsky.

The tycoon, who has been held on charges of fraud and tax evasion, said he was stepping down from the oil company to protect his workforce.

He said he now planned to dedicate himself to his Open Russia Foundation - an organisation he said that is aimed at building an open and democratic Russia.

A BBC Moscow correspondent says the statement indicates Mr Khodorkovsky is preparing to bring forward his declared intention to enter the political arena and challenge those who he believes brought political charges against him.

'Good step'

By resigning from the company, Mr Khodorkovsky is effectively trying to disassociate Yukos from the charges facing him of fraud and tax evasion.

In a statement, he said he "must do my utmost to lead our working team out from under the attack which has been directed against me and my partners".

The fight against Mr Khodorkovsky is a fight against independent people Boris Berezovsky

A Russian stock market analyst, Michael Sito, says traders had widely welcomed the move.

"Investors saw that Mr Khodorkovsky has put a distance between himself and the company which is seen as a very good step," he said.

Many Russians believe the case against Mr Khodorkovsky, who made his fortune through controversial privatisations in the 1990s, is politically motivated.

He has funded opposition groups, breaking what analysts say was a tacit agreement to stay out of politics in return for avoiding investigation of his financial affairs.

'Conservative victory'

The crackdown on Yukos and its boss has stirred fears of a wider confrontation between the Kremlin and big business.

"The fight against Mr Khodorkovsky is a fight against independent people," Boris Berezovsky, a Russian tycoon claiming asylum in Britain, told the BBC.

"[Putin] is now fighting against the most powerful and most independent people."

Analysts have said the Yukos affair ranks as the biggest political and economic crisis of President Vladimir Putin's three-year period in office.

Mr Khodorkovsky's arrest came ahead of parliamentary elections in December, and a presidential election in March.

A BBC correspondent said his decision to step down would be seen by liberals and reformists in Russia as a victory for conservative forces in the Kremlin, who appear to be flexing their muscles ahead of the elections.