Russian billionaire Mikhail Khodorkovsky has been attacked at the jail where he is serving an eight-year sentence, his lawyer says.
Khodorkovsky is in a Siberian penal colony
Yuriy Shmidt said the former head of Russian oil giant Yukos was taken to the Siberian jail's hospital after the attack on Thursday night.
Khodorkovsky needed stitches, Mr Shmidt said, although other reports suggested he suffered only minor scratches.
Khodorkovsky was jailed in May 2005 for tax evasion and fraud.
"Khodorkovsky was assaulted with a sharp instrument," Mr Shmidt said, adding that he was briefly hospitalised and received stitches.
He said Khodorkovsky woke up during the night to find his face covered in blood, having had his face slashed with a cobbler's knife.
But prison authorities played down the attack, saying Khodorkovsky suffered minor scratches during an argument with a prisoner, reports news agency Interfax.
As head of Yukos, Khodorkovsky built up a multi-billion dollar fortune before falling foul of the authorities. He has been imprisoned since his arrest in 2003 along with his business associate Platon Lebedev.
After sentencing Khodorkovsky was sent to a penal colony in Krasnokamensk, in the eastern Chita region close to the Chinese border, about 4,700km (3,000 miles) east of Moscow.
Critics have accused him of ripping off his homeland's natural wealth for personal gain.
His supporters say the case against him was politically motivated and he is paying the price for his political ambitions.