Who is involved
The soldier is a senior serviceman based at Imjin Barracks in Innsworth, Gloucestershire, which serves as the headquarters of NATO's Allied Rapid Reaction Corps. A US official described the soldier as a field grade Army officer, a mid- to senior-ranking officer including the ranks of major, lieutenant colonel, and colonel.
FBI Director Kash Patel said in a statement: "This important arrest involves allegations of espionage, which is of the utmost concern to the United States and our allies."
He added: "While this investigation is continuing, I want to send a clear message – this FBI has no tolerance for anyone who breaks their oath to the United States and whose actions endanger our national security and that of our partners."
Brig Gen Sean Stinchon, commanding general of US Army Counterintelligence Command, said: "The alleged conduct of a senior soldier, entrusted with sensitive information, is a sobering reminder that insider threats are real and persistent."
Deputy Assistant Commissioner Vicki Evans, from Counter Terrorism Policing London, described the arrest as "significant" and said: "The allegations are serious and our investigation is very much ongoing."
Counter Terrorism Policing said the arrest was not linked to any other ongoing investigations.