Theresa May took a costly charter flight to complete her Middle East tour because the official government plane was being used to fly the Prince of Wales on a trip around Europe.
Taxpayers will be left with a bigger bill after the RAF Voyager A330 was secured by Charles and Camilla for the nine-day official visit.
A Clarence House spokesman stressed the royal trip was booked in advance of the Prime Minister's trip to Jordan and Saudi Arabia, which saw her undertake three days of discussion on trade and security issues.
Government ministers and press members meanwhile joined a royal entourage that included the prince's personal doctor, an artist and the Duchess of Cornwall's hairdresser.
Despite being primarily a government plane, the Queen is understood to take precedence in its use followed by Charles ahead of the prime minister and Government ministers.
A Downing Street spokesperson 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."
The Voyager was refitted at a cost of £10 million under David Cameron's time in office to give transport to royals and politicians alike.
Ministers said the conversion would save the taxpayer £775,000 a year in the cost of private charters.
The aircraft is used for air-to-air refuelling when it is not being used for VIP travel.