Opposition parties seized on the revelation, with Labour accusing the Conservatives of spending “disgusting amounts of public money on their own vanity and comfort”, while the Liberal Democrats branded the private jet flight a “staggering waste” of public money.

Critics drew attention to a policy paper co-written by Ms Truss in 2009, called Back to Black, in which the co-authors said all public sector workers should treat the money they spend in office “with at least the care they would give to their own”.

They added: “This change of mindset would be reflected in everyday changes such as travelling by economy rather than business class, to larger scale changes around focusing on value for money.”

Alexandra Hall Hall, a former British ambassador who worked in Government until 2019, said that in her experience: “British diplomats, no matter what their grade, Ambos [ambassadors] included, were always required to fly economy, except flights over 14 hours long, when business class was permitted.”

She added that they were also required to “take public transport to [the] airport wherever possible”.

A spokesman for the Foreign, Commonwealth and Development Office said it was necessary for the Foreign Secretary to travel overseas to “pursue UK interests around security, trade and technology, as she did during this visit to Australia”.

They added: “Travelling this way allows ministers to have private discussions on sensitive security matters and flexibility to respond to rapidly changing global events. This trip used government transport and was fully within rules.”