The GRU is believed to be Russia's largest foreign intelligence agency (Picture: PA).
It is being claimed that the second of two Salisbury nerve agent attack suspects is a military doctor employed by Russia's GRU (Main Intelligence Directorate) intelligence agency.
The two suspects in the attempted assassination of ex-Russian spy Sergei Skripal were originally named by British authorities as Ruslan Boshirov and Alexander Petrov - although it was made clear that the names were aliases.
The Bellingcat group say the suspect identified as Petrov was actually Dr Alexander Yevgenyevich Mishkin, who was employed by Russia's GRU intelligence agency.
Meanwhile, Bellingcat has already identified Boshirov as Colonel Anatoliy Chepiga - a highly decorated officer also believed to be working for the GRU.
The Prime Minister has told MPs the GRU has a "well-established chain of command" and the Salisbury nerve agent attack would have been approved at a "senior level of the Russian state".
What is the GRU?
By Christopher Lee, Defence Analyst
It is the foreign intelligence agency of the Russian military and the biggest overseas intelligence unit in both the military and civilian covert orbit.
In 2010 it was renamed the GU (Main Directorate) - the term indicates the GU’s position in the Russian military set-up.
However, the old term 'GRU' continues to be more widely used.
Unlike other Russian intelligence agencies, such as the Federal Security Services (FSB) and the Foreign Intelligence Services (SVR), the GU does not report to the President.
Its director reports to the head of the minister of defence and Chief of the Defence Staff.
The GU operates special forces (Spetsnaz GU), usually considered to operate on similar lines to the British SAS and the American SEALS.
Spetsnaz is a general term in Russian and former Soviet forces but Spetsnaz GU claims a reputation for excellence, although not one always supported by results.
Military intelligence has formally existed in Russia since the start of the 19th century.
However, the effective modern GU has its origins in 1918 with the formation of the USSR and then in reforms carried out in the Stalinist purges, the restructuring of intelligence after the Great Patriotic War (the Russian name for the Second World War) and more recently with the failure of the Soviet Union, the formation of modern Russia and the establishment of a defence ministry in 1992.
Today it has one guideline: if the military command senses a threat to Russian security, then GU either fixes the problem or knows a man who will.
Recent roles show that the GU makes little effort to disguise its involvement and because it reports to the British equivalent of Secretary of State and CDS and not to President nor Prime Minister, the domestic complications in Moscow are minimal.
The GU is not always blessed with ruthless military efficiency, but its higher echelons in GU headquarters have good memories and never forgive and forget.
For example, when Colonel Oleg Penkovsky gave Soviet secrets to British agent Greville Wynne, both the then GRU and Penkovsky knew the end would be a bloody affair. Penkovsky was picked up, tried, found guilty and then tied to a wooden board and fed alive into the coal-burning furnace of the GRU headquarters.
Trainee GRU officers watched as a reminder that betrayal is never forgiven. Salisbury was supposed to be another version of Penkovsky.
The GU attempted to deliver sentence on Sergei Skripal. Thus far it has failed. It is not over.
There is sometimes a lighter side to the Intelligence Game, even played with the GU.
Years ago, in another trade, one of my tasks was to keep an eye on a GRU officer, a Lieutenant Commander Morzhitsky on the Soviet staff at UN HQ in New York.
Every time we met, he would ask me what sort of nuclear weapon was carried in the Royal Navy’s Sea King helicopter.
Over lunch one day (we had moved to four different tables in the UN restaurant because he was convinced they were bugged – which two of them were) he said he had to have the answer by Friday. He was clearly under a lot of pressure.
I said the Sea King did not carry a nuclear weapon because the aircrews had protested they were too dangerous to fly and one was a member of Campaign for Nuclear Disarmament anyway.
He asked me more about range and payload and I made up some good stuff.
We had a laugh and another bottle and he toddled off to the Russian bubble, in which all their classified stuff was bleached – sent home.
As unlikely as the story was, he must have written it into his weekly report.
Two weeks later he was withdrawn. Pity. His replacement was a smart arse and never invited me to anything.