The Security Service of Ukraine had detained the official, named in reports as Stanislav Yezhov, for acting "in the interests" of the "aggressor" after being recruited by Russian intelligence agencies. He was Groysman's interpreter during his time as speaker of the Verkhovna Rada and later followed him to the Cabinet of Ministers when he became prime minister in 2016.
Yezhov is understood to have worked for Groysman as his interpreter and deputy head of protocol.
"He was respected. And he responsibly and diligently worked for the Russian special services".
Groysman met May at 10 Downing Street on 5 July, when they discussed security, Russia's illegal annexation of Crimea, and Russian sanctions, according to the United Kingdom government briefing paper describing the meeting.
May today (21 December) told a press conference in Poland: "I am aware of the reports in relation to a Ukrainian individual who attended Downing Street earlier in the summer".
'The action that's been taken is a matter for the Ukrainian authorities'.
The BBC's Jonah Fisher posted a photo of Yezhov leaving Downing Street with Ukraine's PM, and said that Yezhov had also met then USA vice president Joe Biden in 2016.
Mr Yezhov is said to have accompanied Ukrainian PM Volodymyr Groysman on a visit, and was photographed with his boss and Mrs May in Number 10.
SBU officers, with the assistance of the government, detained the agent of the Russian special services at his workplace. It supplied him with "special equipment" which was used to collect information about the "activities of [Ukrainian] government structures". The collected data was transferred to supervisors via electronic communication channels.
Yezhov was seen being led away in handcuffs on Ukranian television on Wednesday evening.
SBU investigators are preparing an appeal to an investigating judge with the request to choose custody as a measure of restraint for the detainee. He is likely to face charges of state treason, it added.