Foreign Secretary David Lammy travelled in style last week, using a Government VIP Airbus A321 for his official visit to China.
In contrast, King Charles had to make do with a commercial flight from Heathrow for his trip to Australia.
The monarch flew on a scheduled service to link up with Queen Camilla in Singapore before continuing to Australia.
Upon reaching Singapore, the royal couple switched to an official Royal Australian Air Force jet for their onward journey to Sydney.
David Lammy flew like a King in private jet to China while Charles humbly took commercial flightPAAn insider explained: "It was more appropriate for His Majesty, as Australia's Head of State, to arrive on board a Royal Australian Air Force plane."
This unusual role reversal has raised eyebrows, particularly as the King, who is Australia's Head of State, typically enjoys more luxurious travel arrangements.
The Foreign, Commonwealth and Development Office (FCDO) has defended Lammy's use of the Government aircraft, citing time efficiency and value for money considerations.
They have even threatened legal action over the claims, which were made to The Mail on Sunday by diplomatic sources.
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Prime Minister Sir Keir Starmer and Foreign Secretary David Lammy work on the plane as they fly to Washington DC in September
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A spokesman stated: "The Foreign Secretary's job requires him to travel abroad to promote and protect the UK's interests on the global stage.
"This was the most time-effective way to do that in this instance.
"Value for money is taken into account in all travel decisions and costs are routinely published for transparency."
The luxury flight comes amid denials of reported tensions between the Palace and Lammy's office over a recent deal concerning the Chagos islands.
Both the Palace and the Government have strongly refuted these allegations, with the FCDO even threatening legal action over the claims.
King Charles and Queen Camilla arrive in Australia PA
Pictures are projected onto Sydney Opera House, and reflected in a puddle on the harbourside, after King Charles III and Queen Camilla arrived in Sydney to begin their tour of Australia and SamoaPAEarlier this week, Sir Keir Starmer agreed to the controversial deal which would see migrants arriving at the Chagos Islands deported to St Helena - despite giving up its sovereignty to Mauritius.
The Labour leader's decision bears striking similarities to the Rwanda deportation plan, which he previously denounced as "completely wrong" and "immoral".
Under the new arrangement, asylum seekers reaching the British-owned archipelago on small boats will be sent to St Helena, a remote UK territory over 5,000 miles away in the South Atlantic Ocean.
Migrants will undergo security screening before deportation, with arrivals potentially continuing for up to 18 months or until the Mauritius agreement is finalised.
