Steven Mnuchin, the US treasury secretary, requested the use of a government jet for his European honeymoon, it was reported on Wednesday.

The treasury department’s inspector general is investigating Mnuchin’s request for a US air force plane – with a reported operating cost of around $25,000 per hour – to transport him and his wife, Louise Linton, on their honeymoon to Scotland, France and Italy this summer, according to ABC News.

Donald Trump and first lady Melania Trump and several cabinet members attended Mnuchin’s wedding to British actor Linton in Washington in June.

A spokesman for the treasury told ABC that Mnuchin, 54, a former Goldman Sachs banker, sought the military jet to ensure he had a secure line of communication but aborted the request after it became clear there were other ways to maintain contact.

The spokesman was quoted as saying: “The secretary is a member of the national security council and has responsibility for the Office of Terrorism and Financial Intelligence. It is imperative that he have access to secure communications, and it is our practice to consider a wide range of options to ensure he has these capabilities during his travel, including the possible use of military aircraft.”

Travel on military aircraft is usually reserved for cabinet members who deal directly with national security.

Donald and Melania Trump, along with Mike and Karen Pence, attended the wedding of Louise Linton and Steven Mnuchin in June 2017. Photograph: Kevin Mazur/Getty Images for LS

Democrats seized on the revelations. Ron Wyden, the top Democrat on the Senate finance committee, told ABC News: “You don’t need a giant rulebook of government requirements to just say to yourself, ‘This is common sense, it’s wrong,’. That’s just slap-your-forehead stuff.”

Mnuchin is already facing an inquiry from his department’s inspector general after he and his wife traveled to Fort Knox, Kentucky, at the time of the solar eclipse on 21 August. Counsel Rich Delmar said at the time: “We are reviewing the circumstances of the secretary’s 21 August flight … to determine whether all applicable travel, ethics, and appropriation laws and policies were observed.”

Mnuchin’s office insisted that he was there to attend meetings on tax reform rather than watch the eclipse. The treasury said the couple would reimburse the government for Linton’s travel costs.

But on Monday, Citizens for Responsibility and Ethics in Washington sued the treasury for release of documents relating to the trip. Noah Bookbinder, its executive director, said: “We filed an expedited Foia request because Americans deserve more information to determine whether there has been misuse of government resources.”

Linton, 36, caused further controversy at the time when she posted a photo on Instagram with hashtags of various luxury designers she was wearing. When another user challenged her, Linton went on the offensive: “Aw!! Did you think this was a personal trip?! Adorable! Did you think the US govt paid for our honeymoon or personal travel?! Lololol.” The actor later apologised for her tirade.

She is also well known for a supposed memoir she wrote of her gap year in Zambia. That book later had to be withdrawn from sale after significant inconsistencies were noted and people questioned her account. There was also a sharp backlash to the “white saviour” tropes it trafficked in.

Mnuchin is understood to have stayed for a period at the Trump International hotel, where he was seen in the lobby with a pet dog.