The hefty fine is the largest ever handed out by New York’s Conflicts of Interest Board, capping off a yearslong investigation into the two-term mayor’s use of taxpayer dollars to cover the travel costs of NYPD officers who accompanied him on cross-country campaign stops.

Under the ruling, which he has vowed to appeal, de Blasio, a Democrat, will be forced to reimburse the city for $320,000 spent on the officers’ flights, hotels, meals and rental cars during the four-month campaign. He will also have to pay a fine of $25,000 for each of the security detail’s 31 out-of-state trips, amounting to $155,000.

The order was handed down by the Conflicts of Interest Board Chair, Milton Williams, who found that de Blasio “plainly violated” the city’s prohibition on using public resources to advance a political campaign. De Blasio was advised of this rule prior to his campaign, but “disregarded the Board’s advice,” Williams wrote in his ruling.

In a statement, an attorney for de Blasio, Andrew G. Celli Jr., described the ruling as “reckless and arbitrary,” arguing that recent acts of political violence underscored the security needs of public servants.

“In the wake of the January 6th insurrection, the shootings of Congressmembers Giffords and Scalise, and almost daily threats directed at local leaders around the country, the COIB’s (Conflicts of Interest Board’s) action – which seeks to saddle elected officials with security costs that the City has properly borne for decades – is dangerous, beyond the scope of their powers, and illegal,” he wrote.

De Blasio has faced previous allegations of misusing his security detail. Months before he left office in 2021, a report by the city’s Department of Investigation found he treated the officers as a “concierge service,” using them to move his daughter into an apartment and shuttle his son to college.

De Blasio did consult with the Conflicts of Interest Board about the costs of his security prior to announcing his presidential campaign in May 2019. He was told the salaries for NYPD officers would be covered, but that all other costs associated with their travel would not, the board said.

During interviews with investigators, de Blasio said he did “not have a 100% clear understanding” of the guidance, and “suggested that it was an issue for others to resolve,” according to Kevin Casey, an administrative law judge that recommended the fine imposed by the Conflicts of Interest Board.

Casey accused de Blasio of showing a “deliberate indifference” to the city’s ethics board, then blaming his own employees for the error.

“It is troubling that during his DOI (Department of Investigation) interview respondent repeatedly attempted to shift blame to his lawyers and campaign staff, while failing to recognize his personal responsibility for following the law,” Casey wrote.

In an interview with New York Magazine published earlier this week, de Blasio described his White House bid as a mistake.

“I think my values were the right values, and I think I had something to offer, but it was not right on a variety of levels,” he said. “I think I got into a place of just extreme stubbornness and tunnel vision.”