British authorities are accused of covering up the assassination of a Russian whistleblower who was found collapsed close to his Surrey home in mysterious circumstances.

Surrey police and the family of Alexander Perepilichnyy, 44, believe that the Russian, who moved to the UK in 2009, died of a heart attack in November 2012.

An investigation by BuzzFeed News has now found that authorities in the US and France are increasingly concerned that their British counterparts have failed to properly investigate his death.

"We strongly believe that Perepilichny was assassinated on direct orders from Putin or people close to him," said a senior US intelligence official, speaking to the website.