An account of the beating is included in a 40-page report on Magnitsky's death that the Kremlin's human rights council presented to President Dmitry Medvedev on Tuesday, said Valery Borshchyov, who headed an independent investigation into the death that formed the basis for the report.
The report also lays blame on prison hospital staff and the investigators who jailed Magnitsky for his death.
"Having interviewed the prison doctors and staff, we came to the conclusion that Magnitsky, who was already in bad condition, was beaten in a separate cell," Borshchyov said by telephone.
Magnitsky was beaten in the Matrosskaya Tishina prison, where he was taken for medical treatment for his existing health problems, he said.
"Even after that he wasn't provided any medical assistance," he said.
Magnitsky died shortly afterward, he said.
This is the first time that the beating has been disclosed, said a spokesman for Hermitage Capital. Magnitsky was a lawyer for the Firestone Duncan law firm who represented Hermitage Capital, once the biggest foreign investment fund in Russia. His supporters have long claimed he was tortured in prison, including withheld medical treatment, but no one mentioned the beating.
The Kremlin's human rights council did not mention the beating to Medvedev during a meeting Tuesday because of a lack of time, Borshchyov said.
Instead, council member Mara Polyakova, who briefed the president on the report, said the independent inquiry had determined that the investigators who had charged Magnitsky should never have been involved in the case because of a conflict of interest, Interfax reported.
The investigators had accused Magnitsky of organizing a $230 million tax fraud after Magnitsky accused them of embezzling the money.
In addition, the investigators had lacked sufficient evidence to warrant Magnitsky's arrest, Polyakova said during the meeting in Kabardino-Balkaria's capital, Nalchik.
The council's report does not make a final conclusion on Magnitsky's death because its members are waiting for the Investigation Committee to wrap up its own investigation, said council member Lyudmila Alexeyeva, head of the Moscow Helsinki Group.
"We are continuing to work on the case," said Mikhail Fedotov, the council's chairman and secretary of the Union of Journalists, according to a Kremlin of the Nalchik meeting.
Medvedev said the results of the council's inquiry would be passed to the Investigative Committee, which is working on a separate investigation. The council's report is to be published on its by Wednesday.
The Investigative Committee said Monday that it has identified those responsible for Magnitsky's death but will release their names later. It didn't specify on the date.
Fedotov said some of the council's findings echoed a recent statement by the Investigative Committee that a lack of medical help had directly contributed to Magnitsky's death.
Magnitsky accused Interior Ministry and tax officials of cheating the government out of $230 million in value-added tax refunds. His supporters say his arrest was retaliation by the officials.
Amid an international outcry, Medvedev ordered an investigation into the November 2009 death of Magnitsky, 37. No one has been arrested in connection with the death, and several investigators have been promoted and decorated with service awards.
On Monday, the Dutch legislature called on The Hague to slap sanctions on Russian officials implicated in the case.
The Russian Foreign Ministry reacted Tuesday, calling the Dutch lawmakers' decision "unacceptable pressure" on Russia's judicial system and "an intrusion into the country's internal affairs."
Meanwhile, the Kremlin's human rights council promised on Tuesday to present this fall a report on another case that has tarnished Russia's image — the double conviction of former Yukos CEO Mikhail Khodorkovsky, who is serving a 13-year sentence on tax and fraud charges.
The independent inquiries into both the Magnitsky and Khodorkovsky cases were ordered by Medvedev.
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