Prince Andrew flew to a golf tournament on a nearly $A29,000 (NZ$31,266) taxpayer-funded private jet.
Palace sources say there was no other way for the Duke of York to make the trip last July, according to The Sun.
"Airmiles Andy" boarded a private jet from Farnborough, Hampshire, to The Open Championship in Portrush in Northern Ireland, although there are 13 daily flights from London to Belfast.
The golf club cut ties with former patron Andrew amid the Jeffrey Epstein scandal after his disastrous BBC interview.
"There was no other way for Andrew to make the trip," a palace source claimed.
"He was undertaking a visit on behalf of his patronage. Arrangements in relation to the programme did not enable him to travel by scheduled flight."
And even Princess Anne is under fire for a A$29,700 ($31,954) trip to Rome to watch a rugby match between Scotland and Italy in the Six Nations tournament last February.
Again, a royal source defended the flight, saying: "The royal travel provides support and organises travel for members of the royal family to undertake engagements on behalf of their patronages.
"When we make arrangements for any engagements always mindful of their timetable and how it conflicts with other programs and viability of other forms of transport."
Meanwhile, the Queen is refusing to take a pay rise amid Covid-19 spending cuts across the UK.
Buckingham Palace figures have revealed that the Queen, 94, will get a Sovereign Grant of
A$155 million ($160m) next year, including nearly $A60m ($64m to repair her palaces.
But with a A$63m ($67m) income shortfall as coronavirus affects her estate rents and tourism, the grant will be frozen at $A156 million ($168m) for 2021/22 and 2022/23.
The royal household won't be laying off staff but is trying to save through "efforts and efficiencies".
"Her Majesty understands that families across the country are having to make things go further and that should be the same for the palace.
"The expected shortfalls will not see any calls for extra money but just a determination to be visible and carry on."