Details on troop movements, Europe’s response to Russian operations on the continent and classified intelligence on Ukraine are among military secrets that could have been leaked by a suspected spy from the US Army, experts have warned.

The arrest of a 41-year-old senior soldier based at Nato’s Allied Rapid Reaction Corps (ARRC) is likely to raise suspicion among allies even if no damaging material has been disclosed to Kremlin intelligence agents, analysts said.

More than 400 permanent staff from over 20 countries, including dozens from the US, are based at Imjin Barracks, Gloucestershire, the alliance’s largest UK base and home to the headquarters of the ARRC.

The British-led corps is designed to command land forces ranging from a brigade numbering thousands of troops to more than 100,000 personnel during large-scale operations or exercises.

The i Paper understands that the senior soldier who was arrested on suspicion of espionage for Russia in an operation involving MI5, Counter Terrorism Policing and the FBI is a Lieutenant Colonel.

An officer in this mid-level military rank would generally have access to a broader scope of strategic, operational and higher-level classified material.

However, officials familiar with the investigation told , that no sensitive information was passed to Russia.

Andrei Soldatov, an exiled Russian journalist and Russian security services expert, said the recruitment of a senior US soldier would be a “significant coup” for Vladimir Putin’s intelligence agencies after more than 400 spies were expelled from Europe in the wake of the full-scale invasion of Ukraine.

“It means that someone in the UK could be approached and recruited by Russian intelligence agencies, even after all the mass expulsions which we have been seeing since 2022,” he told The i Paper.

“And given the fact that we are talking about an American national serving on a US base, and it’s Nato, it admits that several organisations now would need to do some damage control, trying to assess what kind of information about operations and having departments could be compromised [if the allegations are proven].

“It is quite costly for the UK and for the US and for Nato.”

David Cattler, a former Nato Assistant Secretary General for Intelligence and Security, told The i Paper the suspected US army spy may have had access to “potentially quite sensitive information about Nato’s military capabilities, operational planning, readiness, exercises and command arrangements”.

Cattler added: “A multinational headquarters also provides insight into how Allies co-ordinate and make decisions. That can be as valuable to an adversary as particular documents or technical information.

“But we should be careful not to confuse assignment with access. What this individual could actually see or obtain would depend on his duties, clearance and need to know.”

Any military information linked to the Ukraine war, Nato’s multinational Allied Reaction Force (ARF) or Europe’s response to Russian operations on the continent would be prized by Moscow.

But valuable detail on how to mount a damaging state-sponsored phishing or cyber attack could also be gleaned with insider knowledge on procedures such as how documents are circulated, or commanders are addressed, said Soldatov.

“What kind of information is shared on what level is extremely valuable. Because after that, you can launch a more ambitious operation targeting, say, top-level officials serving on these military bases, or maybe Nato.”

The UK is a “primary target” for Russian intelligence services, he said, as they really believe Britain organises or conducts operations targeting Russia such as sabotage conducted by the Ukrainians on Russian soil.

“So there is this obsession with the British threat in Moscow, and for them it sounds completely legitimate to say ‘double down on the efforts to do something in the United Kingdom’,” he said.

Earlier this year, Soldatov was sentenced to four years in prison in absentia after a Moscow judge found him guilty of violating Russia’s “foreign agent” law. 

Russia’s intelligence agencies, the FSB, the SVR and military intelligence the GRU had been changing tactics and were no longer relying exclusively on people with diplomatic cover, he said.

“We see people with other European nationalities like Ukrainians and Moldovans, Romanians, and so on and so forth. So Russian passport holders are not the only people who actually could spy for Russia,” he said.

But whereas Russia has paid criminal proxies to carry out hybrid warfare attacks across Europe, including in the UK, for any would-be recruit to ensure they were not speaking to MI5 or the Special Branch, it would be necessary for them to talk to a Russian intelligence operative, Soldatov added.

Asked if espionage targeting Western military installations is increasing, Cattler said: “Russian intelligence has targeted Nato and Western military capabilities for decades. What’s changed is the broader security environment.

“Russia’s confrontation with the West has increased the importance of understanding Nato’s military capabilities and decision-making, while technology has created additional opportunities to identify and approach potential sources.

“I’d distinguish, though, between an increase in hostile intelligence activity, an improvement in our ability to detect it, and an increase in public reporting. Those aren’t necessarily the same thing.”