Labour is demanding Boris Johnson quit immediately as PM because they say he poses a national security risk while he stays in No10.
It follows Mr Johnson’s admission he met a former Russian spy without either officials or his security team present.
Now Deputy leader Angela Rayner and shadow Home Secretary Yvette Cooper have written to new Cabinet Office minister Kit Malthouse demanding a full investigation.
They want MI5 to conduct a damage assessment into a meeting between Mr Johnson and ex-KGB officer Alexander Lebedev at son Evgeny’s luxury Italian holiday home three years ago.
Mr Lebedev senior is sanctioned by Canada over his part in the Ukraine War.
Mr Johnson was Foreign Secretary at the time and Britain was in the grip of the fallout from Vladamir Putin’s Salisbury Novichok attack.
Ms Rayner said: “If the PM is found to have breached national security protocols he has no option but to leave office immediately and face the full consequences of the law.
“He poses an ongoing threat to national security. This murky affair has been covered up for too long already.”
Labour want MI5 to examine the PM’s phone to see if it was bugged and check no secret documents from the NATO summit Mr Johnson had just attended were compromised.
Last week the PM told the Commons Liaison Committee of senior MPs that he had visited Alexander Lebedev on April 28, 2018.
He later handed Evgeny, who owns the Evening Standard newspaper, a peerage.
In their letter the Labour duo tell Mr Malthouse: “He appears to have shown a shocking disregard for national security, raising the possibility of a major security breach right at the heart of government.”
Foreign Office records show the PM arrived for the visit with a mystery guest but left on his own.
Ms Rayner and Ms Cooper want the guest identified. And they also want Parliament’s Intelligence and Security committee to review the findings of the probe.
A Number 10 spokesperson said: “ Boris Johnson, as Foreign Secretary and as Prime Minister, has led the way in standing up to Russian aggression.”