A Russian court has rejected a request from jailed former oligarch and Kremlin critic Mikhail Khodorkovsky for his release, but reduced his 11-year prison sentence by two months.

Khodorkovsky, who was Russia's richest man and the head of oil giant Yukos before being arrested in 2003, will walk free in August 2014, the Supreme Court ruled.

Moscow judges also reduced prison terms for Khodorkovsky's business partner Platon Lebedev.

Reading his appeal via video link from a prison colony near the Arctic Circle, Khodorkovsky, 50, 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."

Khodorkovsky and Lebedev 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.

Terms were to be served concurrently with the existing conviction, meaning they were to be freed in 2016.

Lawyers for the two men appealed and demanded their release, arguing their arrest was illegal and the length of their prison sentences unfair.

Last December, the Moscow City Court reduced the sentence from 13 to 11 years.

Last month The European Court of Human Rights said the trial was unfair and the sentencing unjustified, but dismissed claims it had been politically motivated.