Man regarded by Kremlin critics as political prisoner could be freed on Friday after decree signed with immediate effect

The Russian president, Vladimir Putin, has signed a decree pardoning the jailed tycoon Mikhail Khodorkovsky, the Kremlin has announced.

Putin unexpectedly said on Thursday he would pardon Khodorkovsky, who was once Russia's richest man and is regarded by Kremlin critics as a political prisoner.

The decree pardoned Khodorkovsky on the basis of "the principles of humanism". The Kremlin said the decree took immediate effect. Russian news agencies quoted a lawyer as saying it was possible that the former Yukos oil company chief could be freed as early as Friday.

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 confirmed 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 said they had not heard of any such request.

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.