The chief executive of BP was poisoned in a plot believed to have been orchestrated by the Russian security services, a former employee has told the Telegraph.

Bob Dudley, the American boss of the British oil giant, was forced to flee Moscow after blood test results indicated he was being poisoned slowly, the ex-employee has claimed.

Rumours that Mr Dudley was poisoned began to circulate in 2014, some six years after he left Russia in August 2008. At the time, Mr Dudley was a senior executive in BP, running a highly profitable joint venture called TNK-BP.

But it is claimed by a former employee that the Russian authorities wanted to oust Mr Dudley and put in place a plan to slowly poison him. Ilya Zaslavskiy, who worked for TNK-BP for four and a half years, believes only the Russian intelligence agency the FSB had the expertise to administer a toxin slowly enough to harm Mr Dudley but stop far short of killing him.

The alleged poisoning took place two years after the murder of Alexander Litvinenko, who was exposed to radioactive polonium-210 in London, and a decade before the attempted assassination of Sergei Skripal, who was targeted with nerve agent in Salisbury.