Mikhail Khodorkovsky has been freed, bringing an end to more than a decade behind bars after the Russian president, Vladimir Putin, signed a decree pardoning him.

Putin made the surprise announcement on Thursday that he planned to pardon the country's former richest man, who is regarded by Kremlin critics as a political prisoner. On Friday morning the Kremlin said the decree had been signed. By lunchtime, Khodorkovsky had left the prison camp in northern Russia where he was serving his sentence, his lawyer said. "He has left the [prison] colony. That's all I can say," Vadim Klyuvgant told Reuters.

The decree, signed by Putin, pardoned Khodorkovsky on the basis of "the principles of humanism".

Putin's announcement that he intended to release Khodorkovsky - who has been in jail since 2003 and was due for release next August - came at the end of a four-hour press conference in Moscow.

The former oligarch, who became a staunch Kremlin critic after being convicted of economic crimes in trials that many believe were politically motivated, was seen as a potential political threat to Putin if released. Khodorkovsky has previously said he would not ask for a pardon, as it would imply an admittance of guilt.

However, Putin said on Thursday he had recently received a request signed by Khodorkovsky. He said: "Not long ago he appealed to me for a pardon. He has already spent 10 years behind bars - it's a serious punishment. He mentions humanitarian considerations, as his mother is ill. Given all this, the correct decision should be taken and a decree on his pardoning will be signed very soon."

Lawyers for the former oil tycoon initially said they had not heard of any such request, but it later transpired that Khodorkovsky had been visited by officials in prison and warned both about the deteriorating health of his mother and of the possibility of a new case being prepared against him, and advised to sign.

Comment: Interesting, the initiative came from Putin, though he pretended otherwise!

Khodorkovsky's mother, Maria Khodorkovskaya, told Russia Today television: "It hasn't sunk in yet. Too much has happened since yesterday. I just can't believe it."

Khodorkovsky did not fall under the terms of a wide-ranging amnesty passed by Russia's parliament on Wednesday, in which two jailed members of the punk band Pussy Riot will be released early and the Greenpeace Arctic 30 allowed to leave Russia in the coming days. The sudden bout of clemency has been linked with an attempt to boost Russia's image in the runup to the Winter Olympics, due to be held in Sochi in February. The move towards freeing Khodorkovsky nevertheless comes as a surprise.

When Putin came to power in 2000, he offered an informal deal to Russia's oligarchs - they could keep their wealth but they were not to dabble in politics. Khodorkovsky broke the deal. Keen to turn his company, Yukos, into a modern, international business, he made allegations of corruption in the Kremlin and funded opposition political parties.

Many Russians still regard the oligarchs with distaste, as they made enormous wealth during the 1990s while most of the populace was mired in poverty, but Khodorkovsky's decade in prison has had a redeeming effect. Amnesty International has declared him a prisoner of conscience, and there is sympathy - especially among the new urban middle class - towards his critiques of the Russia's political system. His carefully observed prison sketches are published regularly in the Russian magazine the New Times.

Khodorkovsky has said that if and when he is released, he simply wants to spend time with his family, but many see in him someone who could eventually unite the fractured opposition to Putin.