Boris Johnson showed a “careless disregard for national security” in holding a private meeting with an ex-KGB agent in the wake of the Salisbury poisoning, Labour has said.

Shadow home secretary Yvette Cooper told Sky News’ Sophy Ridge On Sunday programme it was “irresponsible” and “unthinkable” that Mr Johnson, who was then foreign secretary, would have such an encounter without officials being present following the chemical attack on UK soil.

The opposition frontbencher repeated calls for an investigation after Mr Johnson told MPs he “certainly” met Russian oligarch and former Evening Standard proprietor, Alexander Lebedev, in Italy unaccompanied in April 2018.

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This was just one month after the deadly Salisbury Novichok nerve agent attack, which provoked international condemnation of Russian leader Vladimir Putin, who rejected claims of Kremlin involvement.

Ministers initially told the Commons last week that Mr Johnson had reported the meeting to officials “as required”, but issued a clarification within minutes saying the prime minister “thinks” he mentioned it to civil servants.

The controversy has fuelled opposition demands for the prime minister to quit immediately following his resignation as Tory leader and not hang on in Downing Street until his replacement is elected.

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Ms Cooper told Ridge: “I think the issue is about whether you have somebody who is just careless about national security interests. We don’t know what happened at this meeting, we don’t know even whether the prime minister actually declared the meeting properly or told officials about it afterwards.”

She added: “It just feels unthinkable that at a time when we had a chemical attack on UK soil, and it was so serious that we had a NATO meeting about it, about how to respond to Russia, at that time you can have a foreign secretary who would then just go and have a meeting with an ex-KGB agent, someone who has since been sanctioned by Canada for close links to Putin.

“To have that meeting at that time shows a careless disregard for national security risks. We actually need to know the facts about what happened and why on earth he was so irresponsible with our national security at that time.”

Mr Johnson’s former chief adviser Dominic Cummings has also alleged the prime minister met Mr Lebedev at the start of the COVID-19 crisis in March 2020, without officials present.

It comes amid an inquiry by the Commons Intelligence and Security Committee over the appointment to the House of Lords in 2020 of Mr Lebedev’s son, Evgeny Lebedev, the owner of the London Evening Standard and a shareholder in The Independent.

The prime minister has repeatedly denied he intervened to ensure Lord Lebedev was offered a peerage despite being warned by British intelligence the move could pose a security threat.

The peer, whose full title is Baron Lebedev of Hampton and Siberia, has said he has “nothing to hide” and backed the publication advice linked to his appointment.

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