Boris Johnson’s revelation that he met with ex-KGB agent Alexander Lebedev at the height of the Salisbury poisoning scandal suggests a “serious security breach” took place, Labour has said.

Shadow home secretary Yvette Cooper branded the private encounter between the then foreign secretary and Mr Lebedev “unthinkable” given its timing in the aftermath of a chemical attack on UK soil.

During an appearance before the Liaison Committee on Wednesday, Mr Johnson admitted for the first time he had met the Russian oligarch without officials present in April 2018.

Pressure has been mounting on the Prime Minister since then to explain the nature of the meeting – which happened straight after a Nato summit on how to respond to the Kremlin – and whether it was properly disclosed.

Ms Cooper and deputy Labour leader Angela Rayner have now written to Chancellor of the Duchy of Lancaster Kit Malthouse calling for an investigation into the “murky affair”.

Speaking to Sky’s Sophy Ridge on Sunday, the shadow home secretary said: “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.

“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.”

She pointed to the scandal as part of a wider pattern of alleged problematic behaviour from the Prime Minister and said it suggested a “serious security breach” took place.

“I think the problem is that you have somebody who’s still in Downing Street who no-one thinks has any duties, any sense of duty to the country, who’s been responsible for lies, for law-breaking and also today’s allegations around abuse of power towards a young woman while he was Mayor of London,” Ms Cooper told the programme.

“The point you raised in your introduction, the serious security breach that appears to have happened while he was foreign secretary, meeting with an ex-KGB agent straight after going to a Nato summit to discuss Russia at the height of the Salisbury crisis and going without officials, going without any security, apparently going with a guest who has never been disclosed.”

Mr Johnson is set to write to the Liaison Committee about the issue but Labour has said more is needed to establish the facts.

It comes as Mr Johnson’s decision to make Mr Lebedev’s son, London Evening Standard owner and shareholder in The Independent Evgeny Lebedev, a member of the House of Lords is probed by the Commons Intelligence and Security Committee.

The appointment has been shrouded in controversy following a report in The Sunday Times that the peerage was allegedly granted despite a warning from security services that it posed a national security risk after Mr Johnson personally intervened.