Theresa May is having to use a chartered jet to carry out Government business in the Middle East after the Prince of Wales borrowed her official aircraft for his tour of Europe.
Clarence House booked the Prime Minister’s RAF Voyager jet when it organised the Prince’s nine-day tour, and it has been flying, half empty, between European cities.
Meanwhile Mrs May has been using a Boeing 757 operated by the charter firm Air Partner for her three-day trip to Jordan and Saudi Arabia.
When the Voyager was refitted at a cost of £10 million last year the Government announced that it would be available for the use of the Prime Minister, other senior ministers and members of the Royal family.
It has now emerged that the aircraft is operated on a first come, first served basis.
Mrs May has held talks on trade and security with King Abdullah of Jordan and King Salman of Saudi Arabia, trying to negotiate billions of pounds worth of new trade deals, as well as discussing the growing terrorist threat in the region and the civil war in Syria.
The Prince and the Duchess of Cornwall have been conducting soft diplomacy on behalf of the Foreign Office to promote Britain in Italy, Austria and Romania in the run-up to Brexit. It is part of a long-term strategy to use the Royal family to win hearts and minds in the EU.
They are accompanied by an entourage of around 24 people, together with up to a dozen members of the media.
The decision to create a dedicated plane for the use of the Prime Minister was taken by David Cameron, who said it would save the taxpayer £775,000 a year in fees for chartering private jets.
Last November Mrs May took the Voyager to India on a trade mission, leaving the Prince of Wales to use a charter for a trip to Oman, Bahrain and the UAE.
Labour MP Paul Flynn said: "It does seem to me to be outrageous. Of course the Prime Minister should have the first call on the plane – she is the elected ruler - ahead of some unspecified journeys by the heir to the throne."
Downing Street played down the significance of the aircraft swap.
A Number 10 official said: “Ever since we first thought up the idea of the Voyager, it was always designed for use by the Queen and the Prime Minister or anyone attending international events representing the Queen or PM. The Prince of Wales is using it right now as he is representing the Queen on this tour he is on.
“Inevitably, there are clashes in scheduling. On a previous occasion, the PM used it and the Prince of Wales chartered an aircraft. This time, it is the other way round.”