Stanislav Yezhov can be seen walking behind the prime minister in July, as part of the entourage of Ukrainian prime minister Volodymyr Groysman.
- Ukraine's SBU security service arrested an alleged Russian asset this week.
- He has been named in Ukrainian and UK media as Stanislav Yezhov.
- Yezhov was recorded visiting 10 Downing Street this summer.
Video footage shows an alleged Russian spy following Theresa May into 10 Downing Street during an official diplomatic visit this summer.
Stanislav Yezhov can be seen walking behind the prime minister in July, while accompanying his boss, Ukrainian prime minister Volodymyr Groysman.
At the time, Groysman and the British government both had no reason to think Yezhov was anything but a regular aide working for the Ukrainian government.
But he has since been accused of being a Russian intelligence asset, who used his access to the highest echelons of the Ukrainian government to feed state secrets to his handlers.
Footage published by MailOnline shows him walking behind Groysman and May at a meeting in July. He pauses while the two pose for photographs on the steps of 10 Downing Street, then follows them both inside:
MailOnline also published footage of Yezhov arriving at a meeting at Lancaster House in London, followed shortly afterwards by Boris Johnson, the foreign secretary.
Photographs circulated on social media have also shown Yezhov with US Senator John McCain, Canadian Prime Minister Justin Trudeau, and European Council president Donald Tusk.
Yezhov has been named in Ukrainian and British media as the man arrested earlier this week by the SBU, Ukraine's security service.
In a press release, the SBU did not mention Yezhov by name but gave a description consistent with his work.
A government adviser, cited by the Kyiv Post, said Yezhov had been a Russian asset for "at least two years", and that he had been turned while posted to Ukraine's embassy in Washington D.C.
Outlining Yezhov's alleged activity, the SBU claimed that he "collected information about the activities of government structures with the help of special equipment." It did not specify what the equipment was.
Speaking on a visit to Poland on Thursday, Theresa May said she was "aware" of the case, but that it was a matter for the authorities in Ukraine.
A Downing Street spokesman said: "Security at Number 10 is kept under constant review."