AT 3.47pm on Sunday, a man and a woman strolled past a fitness club in the Maltings shopping centre in Salisbury. Roughly half an hour later, the police received a call about a pair discovered unconscious on a bench nearby. The man, Sergei Skripal, had been a military intelligence officer in Russia, where he was convicted of spying for Britain before being exchanged in a prisoner swap in 2010. He and his daughter, Yulia, had apparently been poisoned with an unknown substance. They remain in a critical condition in a Salisbury hospital.

Though the investigation is still in its infancy, suspicion has fallen on Moscow. The case has drawn comparisons with the murder of Alexander Litvinenko in London in 2006. Litvinenko, a former Russian intelligence officer turned critic of Vladimir Putin, died after drinking tea laced with a radioactive substance. An official inquiry in Britain later found that his murder had “probably” been approved by Mr Putin himself. On Tuesday Boris Johnson, the foreign secretary, called Russia a “malign and disruptive force” and threatened that British officials would skip the World Cup in Russia this summer if the Kremlin were linked to the Skripals’ apparent poisoning. Russia has denied any involvement.

Mr Skripal, who had risen to the rank of colonel in Russian military intelligence, was convicted in 2006 of passing classified information about several dozen Russian agents working across Europe to British intelligence. During his trial, Russia’s Federal Security Service (FSB) said that Mr Skripal had been recruited in 1995 and continued to spy for Britain after leaving the Russian armed forces in 1999. The FSB called his leaks as damaging to Russian interests as those of Oleg Penkovsky, a Soviet spy who passed secrets to America and Britain before being caught and executed in 1963. Mr Skripal’s fate seemed to have taken a brighter turn when he was pardoned and released in 2010, swapped along with three others for ten Russian sleeper agents who had been uncovered in America.

If his current misfortune does prove to be a poisoning, it reflects “more cracks forming” in the rules governing the murky world of espionage, says Mark Galeotti, an expert on the Russian security services at the Institute of International Relations Prague, a research outfit. Rather than being a defector like Litvinenko, Mr Skripal arrived in Britain as part of a sanctioned swap. And rather than being a visible critic, he had faded, as old spies tend to, into quiet obscurity. To target him now would be a breach of cold-war etiquette.

Another brazen poisoning on British soil would present several problems for British authorities. For one thing, it would raise questions about Britain’s ability to keep defectors safe. (Former intelligence officers suspect that Mr Skripal may have been reached through his daughter, who travels between Russia and Britain.)

More crucially, it would demand a response from the government. Relations with Russia began deteriorating after the Litvinenko murder, and got worse in the wake of the Ukraine crisis and Russia’s military intervention in Syria. Putting more sanctions on senior Russian officials would probably have little effect on Russia’s behaviour. Measures targeting Russian money and property in Britain might hurt more, but they would require hard-to-muster political will.