Marco Rubio, the US secretary of state, has emerged as a superfan of the University of Florida’s football team, spending more than $1.2m of taxpayers’ money on frequent personal trips to watch them play, a report published Wednesday claims.

Ethics watchdogs have questioned the necessity of the country’s top diplomat shuttling so often between Washington DC and Gainesville, where his son Anthony has a walk-on role as a running back for the team known as the Gators.

An investigation by the Washington Post found that Rubio has attended at least 10 games in the last two seasons, including five this fall, and that of 23 total trips made to Gainesville on military aircraft, 15 were “personal” in nature.

The newspaper said that while Rubio is prohibited from flying commercially, because of security protocols that also cover the secretary of defense and FBI and CIA directors, his use of government transport “appears to significantly outpace that of his predecessors”.

Using published defense department rates listing the cost per hour of different kinds of military aircraft, the Post calculated that the flights alone have cost more than $1.2m.

Most recently, on 7 November, Rubio arrived in Gainesville on board an Air Force C-37, a plane used frequently for the VIP transport of high-ranking government officials, and was seen on the sidelines as the Gators lost 23-7 to Oklahoma. Anthony Rubio played no part in the game.

Rubio, in a statement to the Post, defended his prolific trips to Florida, which often feature flights from Joint Base Andrews in Maryland to the Homestead Air Reserve Base near Miami, and a secondary flight from there to Gainesville.

“I have and will continue to try and make it to as many events that are important to me personally and to my family as possible,” he said.

Statistics, however, show that Anthony Rubio, a senior and the third of his four children, has played in only four competitive games in his four years at the university, two of them this season. According to ESPN, he has a college career total of 14 carries for a combined total of 75 yards, and a single touchdown.

Officials who use military transport are required to reimburse the government the equivalent of an economy-fare ticket for their travel, which in every case would be far exceeded by the real cost of the journey.

Rubio and the state department declined to say how much he has reimbursed the public purse, the Post said.

Cynthia Brown, senior ethics counsel at the nonpartisan watchdog Citizens for Responsibility and Ethics in Washington (Crew), told the Post that Rubio’s travel appeared “more of an optics issue than a violation”, but said it merited further scrutiny.

“If this travel becomes something that is undermining the integrity of your service, are you really taking care of taxpayer dollars the way that you’re obligated to as a public official?” she said.

“It is something that I think is really important to shine the light on.”

Brown noted that while Rubio might not be doing anything that is “technically improper”, he is the sole arbiter of how often and how far he travels.

The Post, citing two former government officials, reported that Antony Blinken, who was Joe Biden’s secretary of state, took personal trips on government aircraft “only a couple of times a year”.

Another Biden official, then transportation secretary Pete Buttigieg, was investigated by the department’s internal watchdog for his use of government jets for personal trips after a complaint by Rubio, who was the Republican senator for Florida at the time.

“American taxpayers deserve assurances that their tax dollars are not wasted by the government’s highest officials,” Rubio wrote in his demand for the review. The inspector general ultimately cleared Buttigieg, who took eight trips at a total cost of $59,000, of any wrongdoing.

Dylan Johnson, assistant secretary of state, said in a statement to the Post that Rubio’s use of military aircraft was to “ensure continuity of operations, continuity of government and access to secure communications at all times”.

He added: “He will continue to do so.”

The Guardian has contacted Crew and the state department for comment.