Up to half the staff at the Russian embassy in London could be involved in intelligence gathering, a senior source told The Daily Telegraph.

Around 40 Moscow spies are believed to be operating in this country at any one time. Some are involved in traditional state espionage, while others monitor London-based oligarchs or engage in industrial spying for the commercial benefit of Russian firms.

There are fears Russia will ramp up its efforts over the coming months while the UK security services focus on the Olympic Games and the Queen’s Jubilee celebrations. Britain’s close relationship with America is also hugely attractive for Russia who sees it as a “back door” to US intelligence, one expert warned.

Relations between the two nations have been strained in recent years, in particular following the murder of Alexander Litvinenko in London in 2006 and difficulties in extraditing the chief suspect Andrei Lugovoi.

Daily Telegraph reports