A ministerial jet converted from an RAF plane to help save money on official trips may have cost more than £150,000 to fly the Prince of Wales and Duchess of Cornwall on their European tour.
Under David Cameron, an RAF A330 Voyager refuelling aircraft was refitted at a cost of £10 million for its new role, and promptly dubbed "Cam Force One" after the US President's jet.
It is designed to provide transport for ministers and members of the Royal Family but can still be used carry out its primary air-to-air refuelling role when 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.
Charles and Camilla flew in the plane for the first time during their tour in March and April, where the prince travelled solo to Romania, then was joined by his wife in Italy and Austria.
The cost of the trip - £154,000 - was revealed in Buckingham Palace accounts with a royal source stressing the figure was the highest of a number of estimates they had for the price of using the plane - but they believed the cost would come down.
The estimated figure was more than double the price of running a charter plane for the royal couple's seven-day long haul trip to Oman, UAE and Bahrain last year, which cost £72,756.
A Clarence House spokesman said: "We're all looking at the Voyager as hopefully a cost effective way to undertake official visits overseas.
"We're all looking very closely at the costs of it and how it is charged because the amount of money has to work.
"But it is on the face of it a very efficient way for the United Kingdom, for the first time I think ever, to have a national VIP aircraft.
"We were up until now I think the only G20 country not to have one, so it's very hopeful that it's there to be called on when it's needed."
Voyager features business class seats at the front of the plane and economy at the back which are sometimes used by travelling media - although it does not have televisions or in-flight entertainment.
During Charles and Camilla's trip their entourage travelled with them including the prince's personal doctor, an artist to capture scenic vistas, and a hairdresser for the Duchess.
The royal couple were 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.
At the time it was claimed Theresa May was forced to take a costly charter flight on a Middle East tour as the prince was using the official Government plane.
But speaking during the trip a Clarence House spokesman stressed the nine-day official visit had been booked in advance of Mrs May's trip.