A Ukrainian interpreter who has been photographed with Theresa May at Downing Street has been arrested on suspicion of being a Russian spy, according to reports.
The prime minister says she is aware of the reports but will wait for Ukraine to come to their own conclusions before commenting.
The man, named as Stanislav Yezhov, was reportedly arrested by the Security Service of Ukraine for acting "in the interests" of the "aggressor" after being suspected of being a Russian spy.
It is claimed Mr Yezhov accompanied Ukrainian PM Volodymyr Groysman on a visit to London in July, and was pictured with Mrs May inside Number 10.
Asked about the arrest, Mrs May said: "I'm aware of the reports in relation to the Ukrainian individual who attended Downing Street earlier in the summer.
"The action that's been taken is a matter for the Ukrainian authorities."
Foreign Secretary Boris Johnson, who is in Russia, reiterated Mrs May's comments, saying: "I really think before we comment on the Ukrainian's Russian spy we should wait for the Ukrainians to come to their own conclusions first. I think the Ukraine should be our first port of call there."
Russia ordered Mr Yezhov to gather intelligence "about the activities of government structures", according to the Ukraine security service.
Mr Yezhov's home and officer were searched by security service personnel. Mrs May criticised Russia for "weaponising information" as she announced a UK-Poland defence treaty.
"The Kremlin is seeking to undermine the international rules-based system, and it will not succeed," she said.
A Foreign Office spokeswoman said: "We are aware of reporting about the arrest of Stanislav Yezhov in Ukraine.
"We can neither confirm or deny the allegations against him. It is not our practice to comment on legal proceedings in other jurisdictions."
- Boris Johnson says we should wait for Ukraine to respond
Asked whether he would raise the issue of the alleged spy during talks in Moscow on Friday, Mr Johnson said: "I think that is a matter for the Ukrainians.
"We want Ukraine to go on the right path and we are worried that Ukraine needs more encouragement, more help, to reform, to make sure it isn't captured by the wrong influences.
"I think possibly what this episode shows is the real difficulties and risks that the Ukrainian government is running. They need a lot of encouragement."