Jailed Russian businessman Mikhail Khodorkovsky has gone on hunger strike over what he sees as the unfair treatment of his ex-business partner.
Khodorkovsky says his actions led to his partner being punished
Khodorkovsky, once Russia's wealthiest man, has refused food and water for several days, his lawyer told Russian TV and radio stations.
Platon Lebedev was placed in solitary confinement last week for allegedly insulting prison guards.
Khodorkovsky was sentenced to nine years in May for fraud and tax evasion.
Lebedev was handed the same sentence. The pair are being held at a Moscow detention centre while they appeal against the verdicts.
Speaking to Russian media, Khodorkovsky lawyer Anton Drel said his client had instructed him to read a statement announcing his protest.
"On August 19... my comrade Platon Lebedev was moved to a 3 sq m [32 sq ft] isolation cell. Platon is seriously ill," the statement said.
"It is obvious that they threw my friend into the isolation cell to get their revenge on me, Mikhail Khodorkovsky, for [giving] articles and interviews."
"He knows he is not alone," the statement added.
"Let the Kremlin think it is showing strength, in fact it is a display of their weakness and fear.
"Being in no condition to enter an open political discussion with me, they use the last weapons - an isolation cell and a common cell."
Mr Khodorkovsky recently gave an interview to a Russian newspaper repeating his belief that his verdict would eventually be overturned.
Lebedev was reported to have refused to be escorted by prison guards on a regular exercise walk in the prison grounds, citing health reasons.
It appears he was accused of insulting his guards in the process of refusing to accompany them outside.
Mikhail Khodorkovsky's supporters protest his innocence of all charges, maintaining that he was the victim of a political campaign to undermine his power base and end his political ambitions.
He has hinted that he may stand for parliament while in prison, a move which prompted prison authorities to move him into more crowded accommodation, says the BBC's Steve Rosenberg in Moscow.