A Russian businessman who was helping Swiss authorities investigate a \$230m (£144m) money laundering scheme in Russia has been found dead outside his Surrey home.
Surrey Police said it was investigating the "unexplained" death of a man in his forties who collapsed in Weybridge on Saturday, November 10.
The Swiss prosecutor's office confirmed 44-year-old Alexander Perepilichny had been assisting authorities with their inquiries.
The Independent reported that Mr Perepilichny was a key witness against the Klyuev Group, a network of Russian officials and criminals implicated in a series of tax frauds and the death in custody of Sergei Magnitsky, a Moscow lawyer hired by Hermitage Capital.
The company was once one of the largest foreign investors in Russia but fell victim to an apparent conspiracy by Russian interior ministry officials and tax officers to defraud the Russian tax system.
Corporate seals were taken from Hermitage Capital following a police raid and used to apply for a series of tax rebates.
The rebates were signed off by courts and tax offices and the money was transferred into a bank which was liquidated shortly afterwards.
Bill Browder of Hermitage Capital claims Mr Perepilichny was a whistle-blower who handed over crucial information on some of the officials involved in the fraud.
He told Sky News: "These documents were definitive and showed the first absolutely evidence based trail of money from government officials who organised this fraud to their own benefit outside the country with money and accounts that could be frozen. It was a huge breakthrough."
The Independent said Mr Perepilichny was the fourth person to be linked to the scandal who had died suddenly.
Mr Magnitsky died aged 37 in November 2009 from acute heart failure in a Russian pre-trial detention facility after being accused of committing the tax fraud himself.
"The Russian government three years into it have covered up and completely protected everybody who was involved in murdering Sergei Magnitsky," said Mr Browder.
A second post-mortem examination has been requested, after the first proved inconclusive, along with toxicology tests on Mr Perepilichny's body.
"The death remains under investigation whilst officers await the results of the post-mortem and toxicology tests, " a spokesman for Surrey Police said.
"Surrey Police continues to work closely with the coroner’s office whilst the investigation remains ongoing."
Mr Perepilichny reportedly sought sanctuary in Britain three years ago and lived in a luxury development in Weybridge.
According to Russian media reports, he was a former business partner of Vladlen Stepanov, whom Mr Magnitsky had named as one of the suspects in the money laundering.