Theresa May has said she is "aware" of claims that a Russian spy who visited Downing Street has been arrested in Ukraine.
The Prime Minister made the admission in a news conference in Poland a day after a picture emerged on social media showing the man in Number 10.
He has been named by Sky's Alistair Bunkall as Stanislav Yezhov, the official translator for the Ukrainian Prime Minister.
Ukraine's security service, the SBU, says he was recruited by the Russians while working abroad.
He previously served in the Ukrainian Embassy in the United States.
Bunkall said he was told Mr Yezhov was in the room with Theresa May for private bilateral talks last July because the Ukrainian PM does not speak good English.
"Bilateral meetings are an opportunity for leaders to be frank and honest with one another in a way they wouldn't be in public. I understand Theresa May and her Ukrainian counterpart discussed sanctions against Russia and British military support to Ukraine, amongst other things.
"It's not inconceivable that Theresa May revealed how far Britain was or wasn't willing to go in its support for Ukraine. This sort of detail could be very valuable to Moscow," he said.
Mrs May, when asked if she was concerned about reports of a Russian spy she was photographed alongside in Downing Street, said: "I'm aware of the reports in relation to the Ukrainian individual who attended Downing Street earlier in the summer.
"The action that has been taken is a matter for the Ukrainian authorities."
The Kyiv Post, Ukraine's main English language newspaper, said Yezhov had been detained on suspicion of espionage for Russia on Wednesday.
He has served as the deputy head of the protocol for the Prime Minister Volodymyr Groysman.
The SBU has opened a case investigating whether he has committed treason after he is suspected of collecting inside information about the activities of Ukrainian cabinet ministers.
He also previously worked for Ukraine's Slovenia embassy and ex-President Viktor Yanukovych, who was ousted in the Ukrainian revolution of 2014, amid allegations of political corruption.
The Ukrainska Pravda news agency claimed government sources said Yezhov had been under surveillance for months.
The meeting that Mr Yezhov attended to translate involved discussions on "the importance of maintaining sanctions on Russia to keep up political pressure until the full implementation of the Minsk Agreement" and "the importance of improving the capability of the Ukrainian armed forces", Downing Street said.
It came as the Press Association reported bizarre scenes in the Commons earlier when a mystery man was ejected from the public gallery for "filming and taking pictures" during a debate on Russian interference in UK politics.