Russian President Vladimir Putin has signed a decree that pardons jailed former oligarch and Kremlin critic Mikhail Khodorkovsky.

The tycoon has already been freed from his prison camp in Petrozavodsk, north-west Russia.

Khodorkovsky, who was Russia's richest man and the head of oil giant Yukos before being arrested in 2003, was due for relaease in August 2014.

The decree said Putin was pardoning for humanitarian reasons, as his mother is ill.

"He has spent over 10 years in confinement – which is a serious term, I believe," Putin told journalists on Thursday.

Khodorkovsky, 50, had described the trial as a sham, and an attempt to "destroy the reputation of the judicial system and trust in this state institution, merely for the sake of extending a prison term for opponents of the authorities."

The oligarch and his business partner Platon Levedev were first convicted for tax evasion in 2005. The trial was widely seen as political, since Khodorkovsky was arrested shortly after falling out with President Vladimir Putin.

Khodorkovsky had been funding opposition political parties and publicly challenged Putin's authority when he criticised the president's handling of corruption within the state during a televised meeting.

In 2010, as the terms of their first sentence were about to expire, Khodorkovsky and Lebedev were handed another 13 years in jail for stealing oil and laundering money.