A court in the Siberian town where Mikhail Khodorkovsky is imprisoned, has ruled that prison authorities acted unlawfully when they placed the former head of Russia's Yukos oil firm in solitary confinement.

Khodorkovsky spent five days in isolation in January for possessing what prison authorities called illegal documents.

The local court in Krasnokamensk ruled Tuesday that the documents were not secret and had been published in the media.

Prison authorities, however, later announced they are returning him to an isolation cell for his own safety after another prisoner slashed him in the face with a knife last week following an argument.

Khodorkovsky is serving an eight-year term at a Siberian prison following conviction on tax evasion and fraud charges. He was the head of what had been Russia's largest oil company.

Supporters of the former oil tycoon have called the charges against him government reprisal for his support of opposition politicians. Kremlin authorities deny this, insisting he is a common criminal.

Russian authorities have filed repeated tax claims against Yukos, effectively breaking up the conglomerate.

Some information for this report was provided by AP.