Speaking at a press conference in Poland, the Prime Minister acknowledged a picture that emerged on social media showing the spy walking behind her in Downing Street.
Mr Yezhov was also pictured last...
He has been named as Stanislav Yezhov, the official translator for the Ukrainian prime minister.
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The SBU said he was detained at his workplace.
Stanislav Yezhov, the official translator for the Ukrainian Prime Minister, is accused of gathering information about government activities.
The SBU said in a statement that the official was recruited by Russian agents while he travelled overseas.
United Nations slaps new sanctions on North Korea
The resolution also bans exports of industrial equipment, machinery, transportation vehicles and industrial metals to the DPRK. North Korea has repeatedly defied Security Council resolutions to halt its nuclear weapons and missile testing.
Aide to the Ukraine interior minister, Anton Gerashchenko, said Yezhov was recruited when he was working at Ukraine's embassy in Washington and acted in Russia's interests for at least two years.
"He enjoyed respect. He worked very responsibly, diligently and creatively for the Russian special services".
SBU claims he was recruited to work for the Russians while working overseas. "The intruder transferred the collected data via electronic communication channels", the press office of the Security Council reported.
The man worked in Ukraine's cabinet of ministers where he would have had access to sensitive government information, as well as attending high-level meetings including with Theresa May and former US Vice President Joe Biden.
Russian Federation and Ukraine have been in a tug-of-war following Moscow's 2014 annexation of Crimea and support for pro-Russia insurgency in eastern Ukraine.
'We have agreed today to bolster our cooperation to counter Russian disinformation in the region. we are both deeply concerned by Russia's attempts to weaponise information, ' she told reporters during a visit to Poland.
Mr Yezhov was detained at his office on Wednesday night. It is not our practice to comment on legal proceedings in other jurisdictions'.
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