The jailed oligarch who was once Russia's richest man has gone on hunger strike to demand Aids treatment for his imprisoned former lawyer.

Mikhail Khodorkovsky, the Yukos oil tycoon, said that he was refusing food and water to highlight the plight of Vasily Alexanyan, the company's former legal chief.

Mr Alexanyan, who is HIV-positive, has accused prosecutors of withholding vital medicines to force him to sign false confessions incriminating Mr Khodorkovsky and his Yukos partner Platon Lebedev in new money-laundering charges that could keep them in prison for another 15 years.

In a letter to Yuri Chaika, the Prosecutor-General, Mr Khodorkovsky said: “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 or defend my rights ... and become the cause of the possible death of my lawyer Alexanyan.

"I can't make a choice. I'm forced to step outside the legal framework and inform you about the start of my hunger strike. I hope very much that the agency you head will make a decision that provides Vasily Alexanyan with guarantees of life and medical care.”

Russia's Supreme Court last week rejected Mr Alexanyan's plea to be released from detention to a civilian hospital during his own trial on charges of money-laundering, embezzlement and tax evasion. He accused prison officials of deliberately keeping him in a damp and filthy cell, knowing that his immune system was destroyed, to wreck his health as punishment for refusing to sign the false confessions.

Russia has repeatedly ignored a request from the European Court of Human Rights to transfer Mr Alexanyan to hospital. Terry Davis, secretary general of the Council of Europe, has expressed concern about his condition in a letter to Russia's representative.

Supporters say that Mr Alexanyan is nearly blind and has suspected tuburculosis. He appeared frail, coughed repeatedly, and claimed to be suffering "hellish torments" as he appealed to the Supreme Court for bail by video link from a Moscow prison.

Mr Khodorkovsky, 44, is serving an eight-year sentence at a prison camp in Chita, Siberia, 3,700 miles from Moscow. Yunus Amayev, head of the region's prisons department, threatened to punish him for going on hunger strike.

"We don't have such a notion as a 'hunger strike'. We have a 'refusal to take food'," Mr Amayev told Interfax News. "If a refusal to take food is not justified by anything, Mikhail Khodorkovsky will be punished for violating internal regulations."

Mr Khodorkovsky was worth an estimated $15 billion as chairman of Yukos when special forces troops stormed his private jet at a Siberian airport to arrest him on fraud and tax evasion charges in 2003. He was jailed in 2005 in a case widely seen as a vendetta waged by President Putin against the oligarch for supporting opposition candidates in parliamentary elections.

Yukos, then Russia's biggest company, was dismembered to pay Kremlin demands for billions of dollars in back taxes. Most of its assets were sold to state-owned companies.

Mr Khodorkovsky would have been eligible for parole last October, after serving half his sentence, in time to challenge Mr Putin at the parliamentary elections in December.

But prosecutors brought fresh charges accusing him of stealing up to $25 billion from Yukos. A court ordered him today to remain in pre-trial detention until May 2, when Dmitri Medvedev is expected to be inaugurated as Mr Putin's successor after the presidential election on March 2.

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