Mark Galeotti, head of the Center for European Security at the Institute of International Relations in Prague and an expert in the Russian security services, wrote in the Moscow Times this week that these factors may suggest a dangerous shift in how Russia is operating abroad. This should challenge “Western states to come up with new ways to respond to and deter these kinds of outrages,” Galeotti argued.

But what should that response be? Johnson has hinted that Britain may respond with some kind of official boycott of the World Cup, due to be held later this year in Russia. It is also possible that Britain could expel Russian diplomats, Calder says, if British intelligence concludes they were involved.

More inventive approaches are possible, too. Britain’s intelligence services might adopt asymmetric tactics, such as releasing information embarrassing to Russia. Bill Browder, a former fund manager in Russia who is a major critic of Putin, suggested using London’s appeal to Russia’s wealthy as a tool to punish the Kremlin.