A Russian supergrass who was helping Swiss prosecutors uncover a multi-million pound money laundering scheme used by corrupt Russian officials has died in mysterious circumstances outside his Surrey home, The Independent can reveal.

Alexander Perepilichnyy, a wealthy businessman who sought sanctuary in Britain three years ago after falling out with a powerful crime syndicate, collapsed outside his mansion on a luxury private estate on the outskirts of Weybridge. He was 44-years-old and was in seemingly good health.

The Independent has learned that Mr Perepilichnyy was a key witness against the “Klyuev Group”, an opaque network of corrupt Russian officials and underworld figures implicated in a series of multi-million pound tax frauds and the death in custody of the whistle-blowing Moscow lawyer Sergei Magnitsky. He is the fourth person to be linked to the scandal who has died suddenly.

Surrey Police said that a post mortem examination of Mr Perepilichnyy had proved “inconclusive” and further tests were being carried out to try to establish a cause of death. The force said the sudden death was not being treated as suspicious but left open the possibility that it will have to carry out further investigations.

Officers were called to his home on a cul-de-sac shared with seven multi-million pound properties shortly after 5pm on a Saturday two weeks ago but the Russian was declared dead at the scene 30 minutes later. Surrey Police would not confirm suggestions that the Russian had been out running.

Evidence provided by Mr Perepilichnyy detailed how a number of tax officials from Moscow became obscenely wealthy in the immediate aftermath of a tax fraud against a British investment fund and used Swiss based bank accounts to purchase luxury properties in Dubai and Montenegro.

Earlier this year Swiss prosecutors opened an investigation into a number of accounts at Credit Suisse after Hermitage Capital Management forwarded them a dossier of evidence implicating the Russian officials.

“Perepilichnyy was the guy who brought all the evidence they needed to open the investigation,” said a source with knowledge of the investigation. “He brought with him records of shell companies, Credit Suisse accounts, property transactions. The whole lot.”

Hermitage Capital was once one of the largest foreign investors inside Russia until it became the victim of a $230m (£144m) tax fraud scheme. Corrupt officials from Russia's powerful Interior Ministry conspired with tax officers to steal corporate seals from Hermitage Capital following a police raid and apply for a series of tax rebates using those seals. Complicit courts and tax offices signed off on the deal and the money was transferred into a bank which was liquidated shortly afterwards.

Hermitage hired the Moscow lawyer Sergei Magnitsky to investigate the scam and he publicly named a number of key Interior Ministry officials who he believed were involved. Days after going public he was arrested by the same men he had accused and was held in prison for a year. He died in November 2009 after being refused vital medication following months of increasingly brutal treatment.

Thanks to tireless campaigning by Bill Browder, the British based CEO of Hermitage Capital, the case has become a major source of embarrassment for Russia and a lightning rod for anger over the precarious nature of international investments in a country where official corruption has become endemic.

Moves have been brought in the EU and United States to place members of the Klyuev Group on visa travel ban lists whilst Senator John McCain has even called on the White House to proscribe the organisation using new powers to curb international crime syndicates.

Russia’s Interior Ministry claims the complex scam was carried out by a sawmill worker and a convicted burglar, both of whom are serving five year sentences, in cahoots with three others who have since died. One died of a heart attack before the crime took place, another was found dead in Ukraine and the third plunged from a balcony to his death.

Hermitage's initial investigation concentrated on the Interior Ministry officials implicated directly in Sergei Magnitsky's death. But in recent months they have published a series of exposes on the new found wealth of a number of tax officials who signed off on the fraudulent tax rebate.

At the time, Hermitage reported that a Russian businessman who had turned on his former colleagues was helping them uncover what had happened to their money.

The British hedge fund declined to comment on Mr Perepilichnyy's death. But a source with knowledge of the investigation in Switzerland confirmed that he had been providing Swiss prosecutors and Hermitage Capital with evidence on the corrupt tax officials.

Today the Swiss Attorney General’s office confirmed to The Independent that Mr Perepilichnyy had given evidence to federal prosecutor Maria Antonella-Bino. Asked whether his death would have a detrimental impact on the investigation the spokeswoman replied: “Our strength resides in our ability to minimise the influence of such a regretful event on our investigation.”