Its reach from the Lubyanka HQ in Moscow, where the KGB interrogated suspects, extends far beyond maintaining security at home.

Though spying overseas is left to Russia’s Foreign Intelligence Service, the FSB can spread its wings. In 2006 it was given the legal right to kill terrorism suspects overseas if ordered by the president. Only on Monday, Mr Putin boasted of how Russia had “thwarted” nearly 500 foreign spies last year.

The FSB’s operations also extend to fake news and cyber hacking which Britain sees as a growing threat.

The mysterious poisoning has echoes of the assassination in 2006 in London of 43-year-old dissident Alexander Litvinenko, a former FSB officer, who died after drinking tea laced with radioactive polonium-210 at a London hotel.

Ten years later a public inquiry concluded that Litvinenko, who was working for British intelligence, had probably been assassinated with the approval of the Russian president.

Russia denied the allegations and made the main suspect, Andrei Lugovoi, a national hero. Mr Litvinenko’s widow Marina told BBC Radio 4: “It’s like deja vu, like what happened to me. “In Russia it is still an old-fashioned and old-style KGB system. “If there is an order to kill somebody, it will happen.”

So far no one has claimed responsibility. Other possibilities include Mr Skripal being targeted by the Russian mafia or business associates.

Since November six senior Russian diplomats have died suddenly, with “heart attacks” or “a brief illness” among the reasons given. They include Russia’s ambassadors to India and the UN and the Russian Consul in Athens, Andrei Malanin, 55.

Last November Russian diplomat Sergei Krivov, 63, was found unconscious and with a head injury on the floor of the Russian Consulate in New York.

On December 19 diplomat, Petr Polshikov, 56, was found dead from gunshot wounds to the head in his Moscow apartment. And on Boxing Day former KGB chief Oleg Erovinkin, 61, was found dead in the back of his black Lexus car in Moscow. The other two were Alexander Perepilichny, 44, and Boris Berezovsky, 67.