ON MARCH 14th Britain expelled 23 “undeclared intelligence officers” from Russia’s embassy in London. The move came after a former Russian spy, Sergei Skripal, and his daughter were poisoned with a nerve agent in the English city of Salisbury. In a co-ordinated show of solidarity, on March 26th America expelled 60 such undeclared spooks. Other British allies, such as Australia and Canada, made similar expulsions. Who are all these undeclared intelligence officers and, if Britain knew who they were, why did it not kick them out sooner?

There are two types of intelligence officers abroad. The first are “declared”: people who officially work in that role, within their home country’s embassy. Almost all countries send such people abroad to act as a liaison between their own intelligence service and that of their host country, for instance in co-operation against terrorism. Host countries are aware of their identity and keep them under constant surveillance. The second type of intelligence officer is “undeclared”. These people are also common in embassies but are not accredited as intelligence officers, because their official job is doing something else, for instance in the consular, political or economic section. As they are not declared intelligence officers, they are theoretically under less surveillance and can do more to gather intelligence. There are, in addition, covert agents embedded in society outside the embassy.

Host countries often turn a blind eye to undeclared intelligence officers within embassies. They will have their own spooks in their embassies abroad, and it is accepted that if you expel such officers, your diplomats and spooks will in turn be expelled, as indeed happened on March 17th when Russia kicked out 23 British diplomats. Host countries often have suspicions about who is a spook among the diplomats. But if a country believes that foreign agents have committed a crime, as happened with Sergei Skripal and his daughter, the only way to punish embassy employees is to expel them, since they have full diplomatic immunity. Covert agents who are not diplomats have no such immunity and so, if they are caught, they may be charged with espionage and put on trial (or, in some countries, much worse).

Intelligence sources suggest that the number of undeclared Russian intelligence officers posted in Western countries has grown substantially in the past decade and that there are at least as many in Britain now as at the height of the cold war. In the short run, the recent expulsions will certainly dent Russia’s ability to gather the intelligence it wants. But the Russians will undoubtedly look to replace their expelled spooks. The latest round of expulsions therefore means at least one thing: Western counter-espionage services will be unusually busy in the coming months trying to identify the new arrivals.