A Russian court has reduced the 13-year prison sentence of jailed oil tycoon Mikhail Khodorkovsky's business partner by three years.

Platon Lebedev is now set to be released in July 2013, 10 years after he and Khodorkovsky were arrested.

They were convicted in 2005 for evading taxes. The two were tried again in 2010 and convicted of stealing oil from their own company and laundering the proceeds. Their case was widely seen as President Vladimir Putin's punishment for Khodorkovsky's political ambitions and support to the opposition.

Khodorkovsky, who was Russia's richest man at the time of the arrest, had funded political parties and civil society initiatives widely seen as challenging the Kremlin.

Amnesty International has declared Khodorkovsky, whose new book is currently leading Russia's non-fiction bestseller list, and Lebedev as prisoners of conscience.

The Kremlin has long insisted the case was not politically motivated. Putin has gruffly dismissed accusations that Khodorkovsky is a political prisoner, claiming that he was "up to his elbows in blood" and saying "the thief should rot in jail."

A court in the northwestern town of Velsk ruled Thursday that Lebedev's sentence should be shortened. Earlier this year, the same court reduced the sentence by a few months, but both sides appealed and a regional court asked for another ruling.

It was not clear whether Khodorkovsky's sentence would be shortened as well. He would need to appeal to a separate court near where he is imprisoned in Karelia, also in northwestern Russia.

Russia's human rights ombudsman Vladimir Lukin said both men should be released immediately, telling the Interfax news agency they had "served the time you get even for the nastiest crimes long ago."

Khodorkovsky was denied an early release in 2008 after a judge cited his refusal to take part in sewing classes in prison and other alleged misdemeanors.

Khodorkovsky is due to be released in 2016.

Last year, then-president Dmitry Medvedev said Khodorkovsky "poses absolutely no danger to society" if released early, but did not seek to commute his sentence.