The Serious Organised Crime Agency (Soca) has been accused of endangering the lives of British residents by passing confidential details to Russian investigators implicated in the death of the whistle-blowing lawyer Sergei Magnitsky.

British businessman Bill Browder and employees of his hedge fund Hermitage Capital have been pursued by Russian investigators ever since they went public about a £144m tax scam orchestrated by a corrupt network of police, judges and interior ministry officials.

Mr Magnitsky was hired by Hermitage's Moscow branch to investigate the scam – the largest in Russian history. He named a network of corrupt officials but was promptly arrested by the same men he had accused. He died nine months later in custody having been beaten and denied vital medicine.

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The case has become something of a cause célèbre in Russia, illustrating the often murky connections between the country's powerful security services and organised crime syndicates. The UN, EU and the US government have spoken out against Mr Magnitsky's death whilst the Kremlin's human rights council claims he was tortured and probably beaten to death.

Despite grave concerns about the investigation it has emerged that Soca forwarded confidential details, including the home address of London-based Hermitage employee Ivan Cherkasov, to Russian officials implicated in the case.

Letters unearthed by a court in Moscow reveal that Soca handed the data to Russia's interior ministry following a request made through Interpol. The move came as a surprise to Hermitage, who say they were told by Soca in 2009 that the Russian requests for information might contravene clauses which forbid the Interpol system from being used for political purposes.

Speaking to The Independent yesterday, Mr Browder accused Soca of endangering his employees' safety. "The Russian interior ministry murdered my lawyer and is now publicly threatening my colleagues in the UK," he said. "I would have expected the British authorities to do everything possible to protect us. Instead they are passing on crucial information to the Russians to carry out their plans. This is either evil or gross incompetence."

Soca refused to comment on the allegations. In a letter to Hermitage seen by The Independent, the agency refused to confirm or deny whether it had handed over details. "Soca is not able to provide categorical assurance that it will refrain from dealing with any requests made by the Russian [or other] authorities in this matter," the letter said.

Mr Cherkasov is a prominent figure in Hermitage's campaign to bring the officials implicated in Mr Magnitsky's death to justice. He lives in London but acts as Hermitage's Russian spokesman, regularly giving interviews to the Russian media in which he has named corrupt officials.

Both he and Mr Browder have received death threats via telephone and text message since moving back to Britain and have notified Soca and the Metropolitan Police. The Met's anti-terrorism unit, SO15, have briefed Mr Cherkasov on security.

Labour MP Denis MacShane has called on the Home Office to tackle what he described as "unconsidered co-operation" with Russian officials in clearly politicised cases.

"While international law enforcement co-operation is an extremely important part of creating a secure environment in the UK, indiscriminate cooperation with regimes and foreign officials, who are well known to be involved in corruption and criminal violence, is something that Soca appears to be doing and doing so without any restraint or oversight," he said.