Theresa May was forced to charter a plane for her recent trip to the Middle East as Prince Charles had already booked the Royal Air Force jet.
The Prime Minister instead flew on a Boeing 747 to Jordan and Saudi Arabia for her three-day visit, while the prince travelled in Europe on the RAF Voyager.
Around £10m was spent refitting the Airbus A330 for the royals and ministers going on the trip with the next in line to the throne.
Clarence House said the Prince of Wales' and Duchess of Cornwall's tour was booked before the PM's trip.
The prince stopped in Austria, Romania and Italy during the nine-day tour, and was joined by his personal doctor, an artist to capture scenic vistas, and a hairdresser for the duchess.
There were also senior members of their household, embassy officials from the countries visited, government ministers, British press and RAF ground crew onboard.
Protocol dictates that the PM is third in line to use the plane after the Queen and Prince Charles.
After that, government ministers can also use the plane for official business.
A Downing Street spokesman said: "The royal visit was organised some time in advance of the PM's Middle East visit, in discussion with the government.
"Two visits at the same time means one aircraft will always have to be chartered."
When the refit took place under previous Prime Minister David Cameron, the government said it would save the taxpayer £775,000 a year on the cost of private charters and could still be used for its primary job of air-to-air refuelling.
"It makes no difference to the public purse whether Voyager is used by a member of the royal family or the prime minister," the spokesman added.