Russian investigators have opened a second posthumous criminal investigation into the whistleblower lawyer Sergei Magnitsky who exposed an alleged £140m fraud by Moscow tax officials, it was claimed.

Mr Magnitsky, who died in a Russian prison in 2009 after suffering beatings and being deprived of medical treatment, became the first dead person to be put on trial in modern Russia when he was last year convicted of tax fraud in proceedings described by critics as evidence of “Sovietisation”. The Kremlin denied the prosecution was an act of revenge to distract attention from corrupt officials but supporters said a further criminal investigation has now come to light, this time accusing Mr Magnitsky of the massive theft which he had himself uncovered.

The death of the 37-year-old auditor opened a new rift between Moscow and Washington, which passed a “Magnitsky Act” banning nearly 20 Russian officials implicated in the lawyer’s death from the United States and threatening to add more senior figures to the list.

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Bill Browder, the British-American financier who employed Mr Magnitsky and has since led the campaign to expose corruption in Russia, said that the lawyer had now been named as the ringleader of four suspects accused of masterminding the $230m (£140m) tax refund theft.

Campaigners said the investigation, disclosed in official papers obtained on behalf of the Magnitsky family, belied efforts by President Vladimir Putin to improve Russia’s international standing ahead of next month’s Winter Olympics by releasing prisoners including the former oil tycoon Mikhail Khodorkovsky and members of the feminist punk band Pussy Riot.

Mr Browder, who runs the London-based investment house Hermitage Capital, said: “If the Russian authorities are steadfastly prosecuting a dead man four years after they killed him, any talk about a Putin thaw from his well-publicised amnesty should be discarded as cynical trash. Everything one needs to know about the real state of justice in Russia can be seen in how Magnitsky’s killers have all gone free and the state continues to desecrate his memory and terrorise his family.”

An official at the heart of the case, Olga Stepanova, who was in charge of the Moscow tax office claimed by Mr Magnitsky to have been at the heart of the alleged fraud, was recently told she will not be prosecuted. Despite evidence of enrichment among officials and investigators linked to the case, no-one has faced charges relating to the lawyer’s death.

The Russian interior ministry carefully denied that the investigation, which appears to have been opened about a year ago but has only now come to light, was a “new” attempt to prosecute Mr Magnitsky, insisting it was not “common practice” in the country to try dead people.

A spokesman told the RIA Novosti news agency: “The information does not correspond to reality. The Interior Ministry’s investigative department... have not started new criminal cases against Magnitsky. There is no such practice in the Russian Federation as instigating criminal cases against dead people.”

The Magnitsky case has proved an international liability for Mr Putin ever since the tax lawyer first uncovered evidence that materials seized from Hermitage Capital’s Russian companies in 2007 were used by an organised crime cartel to take over the subsidiaries and then fraudulently claim the largest tax refund in Russian history.

Mr Magnitsky was arrested in 2008 by the same tax investigators he had accused of complicity in the fraud and held without trial for 11 months. He developed pancreatitis, gall stones and a linked condition for which he was supposed to undergo an operation that never took place. His death in November 2009 came days before the expiry of the 12-month limit on holding suspects without charge. On the eve of his death, he was left untreated in Moscow’s Matrosskaya Tishina prison after allegedly being severely beaten by staff.

In the first posthumous case against him, Mr Magnitsky was convicted along with Mr Browder to have evaded taxes owed by Hermitage Capital, resulting in a nine-year prison term in absentia for the financier, who has denied any wrongdoing, and a refusal by the Russian courts to rehabilitate the lawyer. Under Russian law, posthumous trials are banned unless the proceedings are brought by the relatives of the deceased with the aim of clearing his or her name.

The new case states that Mr Magnitsky had “organised by deceit” the tax refund and “stolen funds from the budget of the Russian Federation and of Moscow City”.

It came to light after Mr Magnitsky’s family filed a complaint to gain access to documents relating to his case which have been withheld by the Russian courts.

Mr Browder said Russian denials of the existence of the new criminal case were motivated by a desire to avoid damaging headlines in the weeks before the winter games in Sochi.