An investigation into whether Sir Keir Starmer broke COVID rules cost Durham Police over £100,000, a Freedom of Information request has revealed.
The Labour leader faced accusations after being pictured drinking a beer in an MP's office in April 2021, with the incident being dubbed "beergate" by the press.
He and his team were later cleared of wrongdoing by the force.
The FOI, put in by National World, showed nine officers from Durham Police's Major Crimes Team spent approximately 3,203 hours working on the investigation.
And the total cost of the probe was around £101,000, funded by the general policing budget - which itself is paid through government grants and local council tax precepts.
The gathering took place in the run-up to a by-election in the city, with Sir Keir, Labour's deputy leader Angela Rayner, and other members of his team on the ground campaigning.
At the time, there was still a ban on indoor mixing between households, but necessary work events were exempt.
The police had cleared the group of breaking COVID rules back in February, but re-opened the investigation after saying they had received "significant new information".
Both Sir Keir and Ms Rayner had pledged to resign their leadership roles if the force found they had broken the law.
After being cleared, the Labour leader said: "People said to me I was taking a risk by saying I would step down if I was fined. But it was never about that.
"For me it was a matter of principle. It shouldn't be controversial to say those who make the law can't break the law. But we have to set the bar far higher than that."
The inquiry took place at the same time the Metropolitan Police was looking into a raft of rule-breaking parties inside Downing Street.
That probe ended with 126 fixed penalty notices being issued, including to the then-Prime Minister Boris Johnson, his wife Carrie, and then-Chancellor Rishi Sunak for attending a birthday party for the PM.