Two plain-clothed police officers were waiting for Dmitry Gudkov as he arrived at London’s Luton Airport last summer. The Russian opposition politician, who lives in exile in an EU country, was flying to the UK to attend a friend’s birthday.

“They were there to intercept me immediately after I exited the plane,” Dmitry says. “That had never happened to me before." But the police weren’t arresting him – instead, they wanted to warn him.

“They told me I’m on a list of people who are in danger. They asked where I’ll be staying and what phone I’ll be using.”

Now, a number of Kremlin critics living in Europe have told the BBC that Russia is stepping up its efforts to silence, threaten and persecute opponents abroad. Some were unwilling to share their stories publicly. The Russian embassy in London didn’t respond to a request for comment.