The Prime Minister will pay the fine issued by Lancashire police, “fully accepts this was a mistake and has apologised”, a No10 representative said.
It is the second time Mr Sunak has received a fixed penalty notice while in government.
He was one of the ministers fined in the partygate controversy, in which government ministers and staff — including former prime minister Boris Johnson — held parties in government offices during the Covid lockdown.
In the latest incident, Mr Sunak was spotted not wearing his seat belt in an Instagram video he filmed to promote levelling-up funding during a visit to the north of England.
“You will be aware that a video has been circulating on social media showing an individual failing to wear a seat belt while a passenger in a moving car in Lancashire,” county police said.
“After looking into this matter, we have issued a 42-year-old man from London with a conditional offer of fixed penalty.”
Mr Sunak has previously apologised for the “brief error of judgment” as he carried out a series of visits in Lancashire on Thursday.
Passengers can be fined £100, and up to £500 if the case goes to court, for failing to wear a seat belt.
There are a few exemptions, including when a car is being used for police, fire and rescue services and for certified medical issues.
But Downing Street did not believe travelling in a ministerial car to be an exception.
In the video, police motorbikes can be seen escorting the car as Mr Sunak addresses the camera.
The government has considered toughening rules to ensure drivers not wearing seat belts will receive penalty points.
Recent Department for Transport figures suggested about 30 per cent of people killed in cars on Britain’s roads in 2021 were not wearing seat belts.
Mr Sunak was also facing criticism for travelling to Blackpool in a taxpayer-funded RAF jet rather than using a train, and then taking a 28-minute flight to Darlington.