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Boris Johnson was accused of being a "threat to national security" after reports he was at a Tory party fundraising event on the night Russian President Vladimir Putin launched his invasion of Ukraine.
The UK Prime Minister gave a short speech at the event as Russian forces prepared to launch their assault.
The Sunday Times reported that donors at the event included Lubov Chernukhin, the wife of a former Russian deputy finance minister.
Mr Johnson reportedly gave an introductory speech at the event, which began about 8pm on February 23 at Spencer House, around a mile from Downing Street.
The Prime Minister was present for about 20 minutes, according to No 10.
Defence Secretary Ben Wallace and Communities Secretary Michael Gove were also at the event, The Sunday Times reported.
Back in Downing Street, at about 4am on February 24, Mr Johnson was woken with the news that Russia had launched the invasion that the West had feared for weeks.
A No 10 spokesman told the newspaper: "The Prime Minister briefly attended this event which was a longstanding diary engagement.
Labour Party chairwoman
"Fundraising is a legitimate part of our democratic process and it is not unusual for the Prime Minister, or any political leader, to attend such an event."
Labour Party chairwoman Anneliese Dodds said: "On the eve of war the Prime Minister should have been completely focused on national security, not trying to get cash from the wife of Putin's former finance minister.
"Boris Johnson's party has accepted over £6.5m from donors linked to Putin's murderous regime. He should be cleaning up our politics, not courting these people for more money.
"Boris Johnson is a threat to national security."
Ms Dodds's comments are the latest sign that Russia and the war in Ukraine will be a domestic political battleground in the run-up to May's local elections.
It comes after a claim from Mr Johnson that Labour would have waved the "white flag" in the face of Russian aggression.
At the Tory spring conference in Blackpool, Mr Johnson said: "Do we want them in charge ... at this moment? Do we want them running up the white flag? Do you see them standing up to Putin's blackmail?"
