Sunak, 42, has apologised for making an “error of judgment” while recording a message for Instagram from the back of an official government car during a visit to northwest England on Thursday.
The Lancashire Police force said it had looked into video “circulating on social media showing an individual failing to wear a seat belt while a passenger in a moving car in Lancashire”.
The force said, without naming Sunak, that it had “issued a 42-year-old man from London with a conditional offer of fixed penalty”.
Failing to wear a seat belt is punishable by a penalty of up to £500 ($AUD889), though fixed penalty notices for such offences are usually £100 pounds ($AUD177) if paid promptly.
The conditional offer means that the person fined accepts guilt but doesn’t have to go to court.
Police didn’t say how much Sunak was fined.
Sunak’s office said in a statement that “the prime minister fully accepts this was a mistake and has apologised. He will of course comply with the fixed penalty”.
It’s the second time Sunak has been fined during his political career.
Last year, when he was Treasury chief, he was fined £50 ($88) for breaching pandemic lockdown rules by briefly attending a party inside government offices.
He was one of dozens of officials fined over the “partygate” scandal, including then-Prime Minister Boris Johnson.
Sunak took office as UK leader in October, promising “integrity, professionalism and accountability” after a tumultuous few years that saw Johnson ousted by multiple scandals and his successor Liz Truss toppled after her policies rocked the UK economy.