As if you needed more proof that Russia has reverted to a dictatorship, the Brits have found conclusive evidence that yet another Putin critic has been assassinated by radiation.

Security sources said MI5 believes the Russian intelligence services assassinated Mr Litvinenko. Britain made a formal request to Moscow for help in the murder investigation. But Mr Putin left diplomats open-mouthed with claims that the former spy did not die ‘a violent death’.

At a dramatic press conference in London the Health Protection Agency (HPA) revealed that he had been killed by a ‘large dose’ of radioactive polonium 210, and not thallium as previously thought.

Only a speck of it would have been enough to prove fatal once it got into his system, probably by being slipped into his drink or on to food.

Whoever did this must have been expert in the dosage because giving him too much would have caused almost instant death while it took weeks for him to become gravely ill, giving the killer ample chance to escape.