Theresa May’s statement saying the UK believes the Russian military intelligence service was behind the Salisbury novichok poisoning shines a further unwelcome spotlight on the most secretive of all the country’s intelligence agencies.

A slow drip of information about operations by the Russian service, known as the GRU, in recent years – from hacking ahead of the US election to support for the Kremlin’s wars in Ukraine and Syria – has shown the wide reach of the agency.

The intelligence wing of the Russian military was renamed the GU in 2010, but both inside and outside the country it is still more commonly known by its old name, the GRU.

The agency, where Sergei Skripal, who was poisoned by the nerve agent novichok, used to work, performs traditional military intelligence tasks and foreign intelligence operations.

For decades Soviet military intelligence kept up a parallel global network of agents and deep-cover “illegals” operating overseas with that run by the KGB. The most famous of the agents was Richard Sorge, who posed as a Nazi journalist in Japan in the 1930s and sent valuable intelligence to Moscow, including details of Adolf Hitler’s plans to attack the Soviet Union, which were ignored by Joseph Stalin.

When the Soviet Union collapsed and the KGB’s foreign spying operations were shifted to a new network, the SVR, the GRU retained a separate status. Although the SVR and GRU often have overlapping interests they tend to work in competition, with separate “residencies” inside Russian foreign missions abroad.

Sergei Tretyakov, who was a high-ranking officer in the SVR’s New York residency until he defected in 2000, explained in a book that there was no overlap between the work of the two agencies. There were two unmarked steel doors on the eighth floor of Russia’s UN mission in New York, said Tretyakov, one of which led to the SVR and one to the GRU. Neither agency had access to the office of the other. The head of the GRU reports to the defence minister and to Vladimir Putin, the Russian president.

The GRU has been identified as the main culprit in hacking ahead of the 2016 US election. A recent indictment from the team of special investigator Robert Mueller named 12 apparent GRU officers over the alleged hacking and leaking of Democratic party emails. Like the US operation, the novichok poisoning fits an apparent pattern of GRU operations: ingenious and audacious, yet uncovered and publicised by the target countries.

The open source investigative team Bellingcat recently claimed it had identified a GRU officer named Oleg Ivannikov as being partly responsible for the downing of Malaysian Airlines flight MH17 over eastern Ukraine in 2014. The officer had also, allegedly, operated under a pseudonym as the defence minister of the Kremlin-backed breakaway state of South Ossetia. Again, it was an operation that mixed impressive tradecraft with errors: among the clues to the man’s identity was a record of an online shopping delivery where he had given his address as GRU headquarters.