The Salisbury poisonings happened after Britain dropped its guard against Vladimir Putin, Gordon Brown has said.
The former prime minister told The Telegraph Magazine that potential targets of the Russian leader were given protection “for possibly years” after Alexander Litvinenko was poisoned in London, but that vigilance levels eventually dropped.
Ten years after Litvinenko was murdered, Russian assassins were able to slip into the UK and make an unsuccessful attempt to murder Sergei Skripal, a former KGB double agent, and his daughter, Yulia. Another woman, Dawn Sturgess, died when she was unwittingly exposed to the nerve agent they had used.
Mr Brown said he had been “under no illusions about what Mr Putin was like” after their first meeting in 2006, when the Russian deliberately made the then chancellor sit in a lower seat than him for a meeting in the Kremlin.
Referring to the death of Litvinenko, a Russian dissident who was poisoned with radioactive polonium-210 in London in November 2006, Mr Brown said he was certain the attack had been authorised by Putin, even though the official report into the killing said it was only “most likely” ordered from the Kremlin.
Warned by security officials that more Putin-sanctioned assassinations were expected, Mr Brown said Britain protected people thought to be “on his list” for years afterwards.
Although Mr Brown does not know whether the Skripals were among those given protection, he said their poisoning with novichok nerve agent happened “when the guard dropped”.
Mr Brown made it clear that no blame should be attached to the police or security services, but said: “You have got to be eternally vigilant about the threat that Putin poses on your domestic soil.”
He added: “The only thing Putin understands is strength. Weakness he will exploit; he’s opportunistic to the nth degree.”
Mr Brown has called for a special tribunal to indict Putin with the crime of aggression, which would be able to act more swiftly than the International Criminal Court in The Hague.
Mr Brown said: “I first met Putin in 2006 at the Kremlin, when I was still chancellor. I was put in a very low seat so that I was looking up at him. He’s certainly a relatively small man, and he wears these stacked heels.
“He took out these index cards, and proceeded to read out all this information he had about me, as though he wanted to prove that he knew more about me than I knew about myself.
“So when people say that Putin’s changed and is only now threatening, I can tell you that he was threatening me even then. I was under no illusions about what Putin was like.”
The Kremlin meeting happened just months before the murder of Litvinenko, a former Russian intelligence officer who served in the FSB, the successor organisation to the KGB.
A British inquiry in 2016 named two Russian spies, Dmitry Kovtun and Andrey Lugovoy, as the likely assassins and concluded the murder was “probably approved by Mr [Nikolai] Patrushev, then head of the FSB, and also by President Putin”. Russia rejected Britain’s extradition requests for both men.
Kovtun died in Moscow in June this year having contracted Covid-19. Mr Lugovoy became a Russian MP in December 2007, a position that enjoys parliamentary immunity from prosecution.
Mr Brown said: “Although the official report concluded it was ‘most likely’ [Putin], I was sure from the information I got that not only had Putin authorised it but that he was about to conduct more assassinations. So we protected the people we knew were on his list for possibly years afterwards.
“It was only when the guard dropped, 10 years later you had Salisbury.”
Mr Brown warned that repeated failure to stand up to Putin’s increasingly aggressive actions had partly led to the war in Ukraine.
The invasion of Crimea, which took place on then-prime minister David Cameron’s watch, “allowed Putin to think he might get away with further incursions”, Mr Brown said.
Equally, the failure of the US to act on the use of chemical weapons in Syria after Barack Obama, the former president, warned Syria’s Bashar al-Assad that the use of such weapons would cross a “red line” was taken by Putin to show the limits of western foreign policy appetites amid complicated military actions in Iraq and Afghanistan.
“We pulled away, we didn’t take action,” Mr Brown said. “Putin saw a weakness in our response. Our mistakes started then.”