MOSCOW - Russian President Vladimir Putin signed a decree on Friday pardoning one of his best known opponents, former oil tycoon Mikhail Khodorkovsky, after a decade in jail.Putin announced unexpectedly on Thursday that he would pardon Khodorkovsky, who was once Russia's richest man and is seen by Kremlin critics as a political prisoner. Regional prison authorities said they could not confirm a report by Interfax news agency that he had already been released from a prison colony near the Arctic Circle.Putin, who appears to be responding to criticism of his human rights record before Russia hosts the Winter Olympics in Sochi, said in the decree that the decision to pardon Khodorkovsky was "guided by the principles of humanity."The decree, released by the Kremlin press service, took immediate effect.Khodorkovsky's lead lawyer, Vadim Klyuvgant, told Reuters the decree "should be enforced immediately" but said it was unclear whether the former Yukos oil company chief could be freed as early as Friday.Putin told reporters on Thursday that Khodorkovsky had asked for a pardon, citing his elderly mother's poor health, and that he would grant it. Khodorkovsky's mother, Marina, and his lawyers said they were unaware of any such request.Khodorkovsky has been in jail since he was arrested in 2003 in what supporters say was part of a Kremlin campaign to punish him for political challenges to Putin, gain control of is oil assets and warn other tycoons to toe the line.He was convicted of grave finiancial crimes in two trials, and his imprisonment has long been held up by foreign investors as evidence of the weakness of property rights and the rule of law in Russia. The Kremlin denies there are any political prisoners in Russia.Yukos was broken up and sold off after Khodorkovsky's arrest and its prize production asset ending up in the hands of state oil company Rosneft, now headed by Putin ally Igor Sechin.