The former vice president of oil group Yukos, Vasily Alexanyan, died on Monday of complications caused by AIDS, media reported.
A Russian right activists said the condition was worsened by poor treatment during a two-year jail term.
Yukos was split up between 2004 and 2007 after its then owner, Mikhail Khodorkovsky, got involved in opposition politics and fell out with the Kremlin.
Khodorkovsky was convicted of fraud and tax evasion in 2005 and Alexanyan was found guilty of acting as an accomplice and served two years in prison from 2006 to 2008.
The European Court of Human Rights criticised Russia at the time, saying it had failed to give Alexanyan proper medical treatment in prison.
“He would still be alive if he hadn’t spent a long time in solitary confinement and had received medical treatment in time,” veteran Russian rights activist Lev Ponomaryov told radio station Ekho Moskvy on Tuesday.
“The correctional system, law enforcement agencies are responsible for Alexanyan’s death,” he said.
Alexanyan’s death adds another dimension to the prolonged legal process surrounding the break up and auctioning of Yukos assets since Khodorkovsky’s arrest on a Siberian airfield in 2003.
Khodorkovsky and business partner Platon Lebedev were found guilty of additional charges late last year in a trial that drew Western criticism and they are due for release in 2016.
A Harvard-trained lawyer, Alexanyan started out as a legal adviser to Yukos, which once pumped more oil than OPEC-member Qatar and in 2006, shortly before his arrest, he was promoted to vice president of the company.
He was diagnosed with AIDS while in jail, where the European Court of Human Rights said he was deprived of potentially life-prolonging, anti-retroviral therapy and went blind.
He was also suffering from advanced cancer, Russian media reported, citing his family.
Alexanyan’s brother Grigoriy said “I confirm (his death) I’m not saying more”, online portal Gazeta.ru reported on Tuesday.
A Moscow court last year withdrew the case against Alexanyan. (Reuters)