Theresa May had to charter a plane for her recent trip to the Middle East as Prince Charles had booked the official Royal Air Force jet, it has emerged.
The prime minister flew on a Boeing 757 to Jordan and Saudi Arabia for her three-day visit, while the prince travelled in Europe on the RAF Voyager.
About £10m was spent refitting the Airbus A330 for royals and ministers.
Clarence House said the Prince of Wales' and Duchess of Cornwall's tour was booked before the PM's trip.
The prince, who stopped in Austria, Romania and Italy during the nine-day tour, 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.
The Queen is understood to get priority to use the plane, followed by Prince Charles.
The prime minister is then third in line, before other government ministers, who can also use it 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.