Mr Khodorkovsky said he was facing an impossible moral choice

Vasily Aleksanian, a former executive of the Yukos oil firm, has AIDS and his health is said to be getting worse.

Mr Khodorkovsky says officials are punishing Mr Aleksanian for refusing to sign false confessions against him.

The two men were jailed in 2005 for fraud and tax evasion in a trial critics said was politically-motivated.

In a letter posted on his supporters' website, Mr Khodorkovsky said Mr Aleksanian had been refused medication and deliberately placed in poor conditions.

He said he had no choice but to "abandon the legal framework" and start a hunger strike.

"I am facing an impossible moral choice: admit to crimes I haven't committed and save the life of a man, but destroy the fate of innocents who will be charged as my accomplices," he said.

Mr Aleksanian has said he has developed serious health complications and is nearly blind.

'Outrageous conduct'

Mr Aleksanian says he believes he will die in prison

Mr Khodorkovsky's international lawyer Robert Amsterdam said Russia was "flouting not only international law but the norms of morality".

In an apparent reference to the Soviet era, he said denial of treatment to Mr Aleksanian returned the Russian prosecutors' office "to its role in a different chapter of Russian history".

"This hunger strike should draw attention to the outrageous conduct of senior members of the Russian Federation who should be held personally responsible for these crimes," he said.

Mr Khodorkovsky, the founder of the Yukos oil company and once Russia's richest man, is serving an eight-year sentence at a prison camp in Siberia.

His supporters have always said that his arrest was punishment for his support of pro-Western opposition political parties.