The group, known as Unit 29155, is believed to have operated for at least a decade, yet Western officials only recently uncovered the true extent of its reach. Intelligence officials in four Western countries say it is unclear how often the unit is mobilised and warn that it is impossible to know when and where its operatives will strike. The purpose of the covert unit is said to carry out missions under orders from Putin to “destabilise the West”.

Hidden behind concrete walls at the headquarters of the 161st Special Purpose Specialist Training Centre in eastern Moscow, the unit reportedly sits within the command hierarchy of the Russian military intelligence agency, widely known as the GRU.

Though much is still unknown about the GRU, intelligence agencies have started to paint a clearer picture of its underlying architecture.

Russian officials have denied the allegations multiple times and Russian Foreign Minister Sergey Lavrov claims he is "astonished" by the "nonsense" claims.

But the intelligence services also report that officers from Unit 29155 travel to and from European countries frequently.

A photograph taken in 2017 shows the man who is believed to be the unit’s commander, Major General Andrei V, posing with Colonel Anatoly V. Chepiga – one of two officers identified over the poisoning of a former spy, Sergei Skripal in Salisbury.

An official from one European security agency told the New York Times in 2019: “This is a unit of the GRU that has been active over the years across Europe.

“It’s been a surprise that the Russians, the GRU, this unit, have felt free to go ahead and carry out this extreme malign activity in friendly countries. That’s been a shock.”

Western intelligence agencies first identified the unit after the failed 2016 coup in Montenegro, but officials began to grasp its specific agenda of disruption only after the poisoning of Mr Skripal.

The Kremlin denied being behind the attack, but it led to a geopolitical standoff, with more than 20 nations, including the US, expelling 150 Russian diplomats in a show of solidarity with Britain.

Intelligence officials believe that a year before the poisoning, three Unit 29155 operatives travelled to Britain, possibly for a practice run.

Speaking to reporters in 2019 at the Munich Security Conference, Alex Younger, the chief of MI6, addressed the growing Russian threat and hinted at coordination.

He said: “You can see there is a concerted programme of activity – and, yes, it does often involve the same people.