British police face questions over the apparent leaking of a businessman's London home address to Russian officials implicated in the suspected murder of a prominent lawyer.

Newly disclosed court documents suggest the Serious Organised Crime Agency (Soca) passed confidential information to staff at Russia's interior ministry, who are accused of being involved in the death of Sergei Magnitsky.

Magnitsky, 37, was working for a British-based investment fund, Hermitage Capital Management, when he exposed a tax fraud worth £144m, the biggest in Russian history. After accusing interior ministry officials of fraud, he was detained in Moscow's Butyrskaya prison, where he died in November 2009 after having had his medication withdrawn. The Kremlin's human rights council claims he was tortured and probably beaten to death.

Now a senior employee of Hermitage – who has already received a number of death threats from Russia – claims his family has been placed in danger by the apparent collusion between UK police and Russian interior ministry officials.

In the two years since Magnitsky's death, senior Hermitage staff have received death threats that prompted them to contact Scotland Yard's counter-terrorism unit, SO15, who offered security in case they were targeted by Russian hitmen operating in London.

In turn, Soca alerted Interpol with its concerns about the case and warned that any extradition request by the Russian Federation or other attempts to arrest Hermitage staff "should be declined".

Yet despite the official assurances that it would not co-operate with the Russian authorities, evidence has emerged suggesting that Soca has leaked the London address of Hermitage executive Ivan Cherkasov – the subject of not only death threats but an arrest warrant from a Russian official implicated in the alleged tax fraud – to the country's interior ministry. One document says: "Based on the information from the National Central Bureau of the UK [part of Soca], the above stated individual [Cherkasov] is residing at his home address [details enclosed]."

The documents were sent from staff at Russia's interior ministry to an individual named as among those who, it is claimed, prevented Magnitsky's lawyers from seeing their client shortly before his death. The same official is part of a group of 60 Russian officials banned from entering the US for their alleged involvement in Magnitsky's death.

The disclosure comes amid mounting concern over targeted assassinations carried out by Russians in London and follows the recent attempted murder of wealthy Russian banker German Gorbuntsov, who was about to give evidence about an attempt on the life of another Russian banker when he was shot five times outside his Canary Wharf home.

Reports subsequently emerged that MI5 had uncovered a plot to murder the prominent London-based Chechen dissident, Akhmed Zakayev.

Last week lawyers for Bill Browder, chief executive of Hermitage Capital Management, wrote to Soca demanding answers to the UK providing information that could be used by Russian "gangsters" to target his colleagues.

The letter from law firm Peters & Peters to the director general of Soca, Trevor Pearce, demands clarification of how sensitive and potentially life-threatening information was handed over despite promises to the contrary.

It states: "Provide your justification in acquiescing to any request for information from the Russian authorities in this case, given our concern about the motivation of the Russian authorities."

The letter also requests details of other information provided to the Russian authorities and urges that the co-operation should cease.

Browder, who has campaigned to have Magnitsky's killers brought to justice, said: "It is incredible that there has been collusion between British police and individuals who are effectively criminals."

Last month MPs passed a binding resolution calling on Britain to ban and freeze the assets of the officials linked to the Magnitsky case. Foreign Office minister David Lidington said the government is "carefully considering" the call. Soca declined to comment.