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People who held birthday parties in breach of Covid lockdown laws have faced fines of more than £10,000.
Met Commissioner Dame Cressida Dick said the investigation was launched after the Cabinet Office inquiry team, led by civil servant Sue Gray, passed information to the force.
Ms Dick's investigation heaps further pressure on Boris Johnson, who is facing fresh calls to resign after it emerged that he had a birthday party in the Cabinet room during lockdown in 2020.
The prime minister has welcomed the probe, saying it would "give the public the clarity it needs" over the allegations.
A separate inquiry being conducted by Ms Gray will continue but its findings are unlikely to be published until the Met has finished its work.
Revelations about the PM's birthday bash sparked a fresh wave of anger among members of the public who were unable to visit loved ones during the pandemic.
Others who share a birthday with Mr Johnson said they were "disgusted" by news of the gathering.
A single mother born exactly a year before Mr Johnson spent that day at home in Winchester, Hampshire, with her 25-year-old daughter Kirsten, who was recovering from a thyroid cancer operation and had a "7.5-centimetre gash" across her neck.
"It was terrible, absolutely awful... we're sitting there holding each other - but hardly holding each other because she was so sore with her stitches... we pulled ourselves through it completely alone," Donna Levermore, 58, she said.
"(Mr Johnson) has infiltrated and spoiled everything - literally stomped all over it... I'm disgusted. He's tarnished my bloody birthday.
"My daughter lost her neck... and this b******d is dancing the fandango."
Enfield resident Torino Reid was one of those who was fined for breaking the rules.
He was hit with a £14,000 charge after police broke up a gathering in the shed in his back garden, which had been organised to mark his niece’s birthday, the Evening Standard reported.
PC Gareth O’Connor identified 44 people present for the celebration, on 27 December 2020, with disco lights, music playing, and bottles of alcohol.
In a statement to court the officer accused the guests of “significant disregard for both the restrictions as well as the genuine danger and threat of coronavirus”.
University student Ruben Alfredo was given a £10,000 fine for holding a birthday party for himself at his cousin’s home in South Kensington on 31 May last year, the Evening Standard added.
“It was evident that a party had been going on, and that a large group of people were in attendance. There were bottles of alcohol, plastic cups, and decorations littered across the premises”, said PC James Nelson.
Angus Proud from Barrowford in Pendle, Lancashire also shares a birthday with the PM and said it was "galling" to hear the allegations that Mr Johnson had celebrated it while others were following restrictions.
Mr Proud tweeted a picture on Monday of himself enjoying a quiet birthday drink on June 19 2020, which he spent with his partner.
"On June 19 2020, I had a night out(doors) for my birthday. Just the 2 of us," he tweeted.
Mr Proud, 68, said: "I was never a fan of Johnson, but I did think in March and April that year that he got the country on the right path.
"To find out now that he was leading his own life by his own rules is pretty galling. We had a pleasant enough day, but not the day we could have had.
"And that's fine when you know we're all in it together. "But we weren't, were we?"
Mr Proud said that he "absolutely" believed he was doing the right thing by having a quiet birthday, and said "it felt like a national effort to us".
He added: "Usually I would be doing something on my birthday, nothing big, maybe a meal out with family or friends, but in 2020, obviously, we were all following the rules.
"The two of us went for a stroll locally, had a few phone calls and then dinner for two at home."