British citizen Paul Whelan, who was arrested in Moscow on suspicion of spying last month, has been accused of gathering information about a Russian military structure and was being monitored by security services here since May, The Telegraph can reveal.

Mr Whelan was charged with espionage, which carries 10 to 20 years in prison, but details of the case against him have not been announced.

Investigators believe that Mr Whelan was gathering information about “classified military structures” and have recordings of his phone calls and online chats, a source close to the case said.

The GRU, the military intelligence agency accused of poisoning Sergei and Yulia Skripal with a nerve agent in Salisbury in March, could fit this description.

Security services were monitoring Mr Whelan, who has British, American, Canadian and Irish citizenship, since he made a trip to Russia in May. He was meeting with a friend from VK, the Russian social network, in the Metropole hotel near the Kremlin when he was arrested by FSB security agents on December 28, the source said.

More than 20 of his friends on VK completed a military education or service. Mr Whelan was discharged from the US Marine corps for bad conduct and was head of security for an auto parts company in Michigan. He had traveled to Russia numerous times since 2006.