Theresa May forced to charter plane because Prince Charles was using her £10 million RAF jet

Prince Charles visited Romania, where he has business interests in tourist cottages

Prince Charles makes a toast during the three-country tour (Getty Images)

The Prime Minister was forced to charter a private jet for her diplomatic visit to the Middle East because Prince Charles and his entourage had booked use of her official RAF plane for an overseas trip of their own, it has emerged.

Prince Charles and the Duchess of Cornwall went on a three-country tour of central Europe – including to Romania, where Prince Charles has business interests in tourist cottages.

The prince owns and runs the Zalánpatak eco-farmhouses in Transylvania in a joint venture with Count Kálnoky, a local aristocrat and conservationist.

Clarence House said because the prince had booked his trip before the Prime Minister’s he was entitled to use the RAF Voyager A330, which was refitted at taxpayer expense for £10 million and was supposed to save money on chartering private aircraft for official visits.

David Cameron’s administration had claimed that taxpayers would save £775,000 a year on the cost of VIP flights.

It is however understood that the Queen and Prince Charles have first priority to use the new plane ahead of the Prime Minister.

Theresa May visited the Middle East and Persian Gulf to woo governments, including the brutal Saudi Arabian autocracy.

The Prince and Dutchess of Cornwall went on a nine-day tour of Austria, Romania and Italy. They were joined by a personal doctor, an artist to capture scenic vistas, and a personal hairdresser for the duchess.

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.”