Theresa May was forced to take a costly charter flight on her Middle East tour after the Prince of Wales took the government's RAF Voyager for a trip around Europe.
The Prime Minister has landed back in London after three days of discussions about trade and security issues.
But taxpayers will be left with a bigger bill after the official government plane was secured by Prince Charles. A Downing Street spokesman, however, 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.
"It makes no difference to the public purse whether Voyager is used by a member of the Royal Family or the Prime Minister."
A Clarence House spokesman stressed that the nine-day official royal visit was booked in advance of Mrs May's trip to the Middle East.
The Prince's entourage includes his personal doctor, an artist to capture scenic vistas, and a hairdresser for the Duchess of Cornwall.
The royal couple was also joined on the RAF jet by senior members of their household, embassy officials from the countries visited, government ministers, British press and RAF ground crew.
But many seats on the Voyager have been left empty during the tour, which included stops in Romania and Italy. Graham Smith, chief executive officer of anti-monarchy campaign group Republic, said:
"I think there should be no question that the Prime Minister should have precedence. She's doing a serious job of governing the country and Prince Charles is doing something which isn't going to help anybody."
He added: "I think that the government should clearly have precedence because they're doing a serious job of government.
"If the royals are doing an official engagement, they need to use whatever transport the government wishes to provide for them, and they need to stop using official transport for non-official purposes."
Mrs May has spent three days visiting Jordan and Saudi Arabia on the Boeing 757.
Under her predecessor, David Cameron, one aircraft from the Voyager fleet was refitted at a cost of £10 million to provide transport for ministers and members of the Royal Family.
Ministers said the conversion would save the taxpayer £775,000 a year in the cost of private charters, while the aircraft would still be available for its primary air-to-air refuelling role when it is not being used for VIP travel.
It is understood the Queen takes precedence in the use of the plane followed by Charles, the Prime Minister and then government ministers.