Lancashire Police issued the notice after the prime minister appeared without a belt in a clip on Instagram as he promoted his levelling-up funding in Lancashire.

The PM has accepted the fine.

A Number 10 spokesperson said: "The prime minister fully accepts this was a mistake and has apologised. He will of course comply with the fixed penalty."

PM admits mistake and will pay fine - politics latest

The force 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 issued a 42-year-old man from London with a conditional offer of fixed penalty."

Mr Sunak is the second serving prime minister - after Boris Johnson - to be found to have broken the law while in office.

The prime minister previously apologised for the "brief error of judgment" as he carried out a series of visits in Lancashire on Thursday to announce the latest wave of levelling up funding.

In the clip, police motorbikes escort the car as Mr Sunak addresses the camera.

Mr Sunak has previously been fined by the Met Police for breaking lockdown rules.

He received a financial penalty alongside Mr Johnson from the Metropolitan Police for attending a gathering to mark the then-PM's 56th birthday - in spite of the coronavirus restrictions in place.

Sunak 'a total liability'

Labour said the prime minister has been turned into a "laughing stock".

A party spokesperson 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."

The party's deputy leader Angela Rayner added: "Rishi Sunak is a total liability."

Fellow Labour MP Sir Chris Bryant said: "It's time they all went. It's time for a general election."

"When Rishi Sunak arrived in Downing Street, he said that his government was going to be of professionalism and integrity. Well this is neither professional, nor does it show any integrity whatsoever," the MP for Rhondda said.

"We have already got Rishi Sunak fined last year, now he's been fined again. This is more than accidental; it shows a pattern of misjudgement."

While the Liberal Democrats said the PM had "shown the same disregard for the rules as Boris Johnson".

Lib Dem deputy leader Daisy Cooper said: "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."

'Fixed penalty notice isn't a criminal offence'

Sky News' policing analyst Graham Wettone said he is "not surprised" that the PM has been issued with a fixed penalty notice, adding that it is the "right and proper" resolution.

"I am not surprised. The prime minister has committed an offence, Lancashire Police have looked at it and decided actually he needs to be issued with a fixed penalty notice.

"So it is exactly what would happen to anybody else if they were to commit a similar offence and the police discovered it."

Mr Wettone dismissed criticism that the PM has been treated unfairly, suggesting the case was "fairly simple" and that the law must be applied "impartially".

But Conservative MP Scott Benton attempted to play down the significance of the PM receiving a fixed penalty notice.

"This is something millions of Britons experience every single year," the Blackpool MP said.

"The Cabinet Office set out clearly last year that a fixed penalty notice isn't a criminal offence.

"Rishi Sunak's been in government for a number of months now. He has got an ambitious agenda to deliver on the priorities of the British people."

Mr Benton yesterday suggested the investigation by Lancashire Police was "not good use of frontline resources".

Sunak's early premiership hit by controversies

Earlier, Downing Street said Mr Sunak "fully accepts this was a mistake and apologises".

A Number 10 spokesman said "of course" the prime minister does not believe anyone is above the law, adding that he "believes it is important for people to wear their seatbelt".

"It was an error of judgment, he removed it for a short period of time to film a clip, which you've seen, but he accepts that was a mistake," the spokesman added.

Fines of up to £500 can be issued for failing to wear a seatbelt when one is available.

There are a few exemptions, including when a car is being used for police, fire and rescue services, and for certified medical issues.

Mr Sunak came to office promising "integrity" after the scandals that eventually forced Mr Johnson from office.

Despite only being in office since October, Mr Sunak's premiership has already been hit with a set of controversies, from criticism for reinstating Suella Braverman as home secretary six days after she was forced to step down over a security breach to an ongoing bullying inquiry into Dominic Raab, the deputy prime minister.