"It is one thing to work in a partially free country, as Russia was six years ago. It's another when the country has stopped being politically free," Illarionov, 44, was quoted as saying by ITAR-TASS news agency. He said he expected Putin to accept his resignation.

Illarionov has long been the highest profile critic within the Kremlin of the Putin administration, in particular slamming the break-up of the giant oil company Yukos and imprisonment of its founder Mikhail Khodorkovsky on what critics called politically motivated charges.

He described the Yukos takeover in 2004 as the "scam of the year."

Analysts said that Illarianov had already lost influence in the Kremlin inner circle, but was kept on as a liberal figleaf to ease Western concerns about growing state control over the economy and Putin's domination of the political system.

"Illarionov was largely window dressing for the administration, a symbol of the liberalism and pluralism that are insufficient in real life," Indem foundation director Georgy Satarov told Echo Moscow radio. "I think he was sick of fulfilling this role."

Mikhail Delyagin, head of the Institute of Globalisation Problems, told Echo Moscow that the decision to quit could not have been easy. "People who split with their boss by their own free will usually become personal enemies of that boss, and with the current president that is not only unpleasant, but dangerous."

In his resignation comments, Illarionov said that Russia had become unrecognisable from the country he knew when joining the Kremlin as then acting-president Putin's economic adviser in 2000.

"In these six years the situation has radically changed and in the last year it became clear that not only the political, but economic model of the country has changed," he was quoted as saying by ITAR-TASS. "I did not go to work for such a country, or sign a contract, or swear an oath."

"As long as I could do at least something, including talking, I thought it was important to stay," he was quoted as saying by RIA-Novosti.

Illarionov has cut an increasingly lonely figure as he lashed out at the direction the country was taking -- in stark contrast to the rest of the Kremlin's tightly managed information machine.

Just last week, in what may have been his last major press conference as adviser, he said: "Russia has ceased being a free and democratic country."

In January of this year he lost his job as the Kremlin's point man on relations with the influential Group of Eight (G8), which groups Britain, Canada, France, Germany, Italy, Japan, Russia and the United States. Russia takes over chairmanship of the G8 next year.

His removal from that dossier was widely viewed as punishment for his denunciation of the Yukos affair.

Seen as one of the brightest and most liberal Russian economists, Illarionov obtained a PhD in economics from Leningrad State University. He joined as a government economic adviser in 1992, and became an adviser to then-premier Viktor Chernomyrdin, before being dismissed in 1994.

In 1999 he helped elaborate an economic programme for Putin who was being groomed to take over from then-president Boris Yeltsin.

Once in the Kremlin, he repeatedly took an independent line in Moscow politics, notably opposing the government's support for the Kyoto protocol on global warming, which Russia ratified in 2004.

He challenged the scientific and economic data on which the treaty is based and said it would lead to a fall in economic growth.

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