Rishi Sunak was fined by police last night after he posted a video to his Instagram showing him not wearing a seatbelt.
Downing Street said the Prime Minister will pay the fine and “fully accepts this was a mistake and has apologised”.
He becomes the second PM found to have broken the law in office after Boris Johnson was fined for Partygate.
It is also the second fixed penalty notice Mr Sunak has received in less than a year.
He was fined by police in April for attending a lockdown-busting gathering in No10 to celebrate Boris Johnson's 56th birthday when he was chancellor.
In his first speech when he became PM in October, Mr Sunak pledged a premiership with "integrity, professionalism and accountability at every level" but that promise has now been thrown into disarray.
Downing Street said the PM had made a "brief error of judgement" by removing his seatbelt as he recorded a video to promote levelling up funding as he visited Lancashire on Thursday.
Mr Sunak was criticised after uploading the footage to his official Instagram account, which has 1.3million followers.
He was forced to make an embarrassing apology for the gaffe.
Wearing a seat belt is a legal requirement for all drivers and passengers. Fixed penalty notices for seat belt offences are usually £100, but they can rise to £500 if taken to court.
No10 say they don’t know how much Mr Sunak's fine is yet as he hasn’t officially received it.
A Labour spokesman last night said: “Hapless Rishi Sunak’s levelling-up photo op has blown up in his face and turned him into a laughing stock.
“He started the week hoping people would be grateful for a partial refund on the money that has been stripped from them over 13 years of the Tories. But instead he got a warring party and yet another fine from the police.
“Just when you thought this Tory government couldn’t get any more ridiculous, they manage it.”
Deputy Leader of the Lib Dems Daisy Cooper said: "Rishi Sunak has shown the same disregard for the rules as Boris Johnson, and now becomes the second ever Prime Minister to be fined by the police.
“From partygate to seatbelt gate, these Conservative politicians are just taking the British people for fools.
“Whilst they continue to behave as though it’s one rule for them and another for everyone else, this fine is a reminder that the Conservatives eventually get their comeuppance.”
Labour MP for Lancaster and Fleetwood Cat Smith thanked Lancashire Police for "reminding us no one is above the law".
Former Shadow Transport Secretary Lilian Greenwood said: “Not only is failing to wear a seatbelt against the law but deeply irresponsible.
“More than a fifth of those killed in cars were not wearing a seatbelt - and not just a danger to themselves but to others in the car. What is it with Tories and a total disregard for road safety?”
Former Tory MP Christian Wakeford, who defected to Labour last year after losing faith in the Conservatives, pointed out that Mr Sunak was “surely the first person to break the law as Chancellor and Prime Minister”.
He said: “As ever it’s one rule for Rishi and his wealthy mates and another for the rest of us.”
Labour MP Dawn Butler added it “feels like you can only become a Tory Prime Minister if you're willing to break the law”.
Lancashire Constabulary, who issued the fixed penalty notice, said: "You will be aware that a video has been circulating on social media showing an individual failing to wear a seatbelt while a passenger in a moving car in Lancashire.
"After looking into this matter, we have today (Friday, January 20) issued a 42-year-old man from London with a conditional offer of fixed penalty."
A No10 spokesperson said: "The Prime Minister fully accepts this was a mistake and has apologised.
"He will of course comply with the fixed penalty."
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