Rishi Sunak has been branded a "total liability" and "hapless" as he faced a backlash for being fined for a second time, with police handing him a fixed penalty notice over not wearing a seatbelt.
Mr Sunak has accepted the fine, with Downing Street saying he "fully accepts this was a mistake" and will "of course comply with the fixed penalty".
Critics are now accusing the PM of showing "the same disregard for rules" as his Number 10 predecessor Boris Johnson.
Labour deputy leader Angela Rayner tweeted: "Rishi Sunak is a total liability."
Sir Chris Bryant, Labour MP for Rhondda, said: "Sunak promised honesty, integrity and accountability on the steps of Number 10. Not only has he been fined again for breaking the law, but Zahawi has been fined as well. It's time they all went. It's time for a general election."
A Labour spokesperson added: "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."
'No one is above the law'
Labour MP Cat Smith thanked police for their road safety work before taking a swipe at Mr Sunak by saying "no one is above the law".
But Tory MP Scott Benton struck a different tone to his fellow Lancashire MP, saying the seatbelt complaint was politically motivated and bad use of police time.
In an apparent bid to downplay the significance of the notice, the Blackpool South MP tweeted: "@LancsPolice do an amazing job, but I'm sure their time is better spent investigating serious crime which impacts on my constituents.
"The vast majority of people would think that politically motivated complaints about a seat belt are not good use of frontline resources."
Meanwhile, Liberal Democrat deputy leader 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."
The criticism comes as Labour is also calling on Mr Sunak to dismiss Nadhim Zahawi from his cabinet after questions over the Tory chairman's tax affairs remain unanswered.
Questions have swirled following an article in The Sun on Sunday, which claimed a seven-figure payment was made by Mr Zahawi to end a dispute with the taxman "after scrutiny of his family's financial affairs".
Second serving PM to break the law while in office
Mr Sunak is the second serving prime minister - after Boris Johnson - to be found to have broken the law while in office.
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."
He and Mr Johnson were fined by the Metropolitan Police over a birthday party held in Downing Street for the former prime minister when curbs were in place.
Law must be applied 'impartially'
Sky News' policing analyst Graham Wettone said he is "not surprised" about the notice, adding that it is the "right and proper" resolution.
"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 prime minister has been treated unfairly, suggesting the case was "fairly simple" and that the law must be applied "impartially".
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.
Mr Sunak's premiership has been hit with a series of controversies since he entered Number 10 in October, 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.