LIZ Truss has come under fire for taking a private "Union jet" on a £500,000 flight to Australia in a "grotesque misuse of taxpayers' money".

The Foreign Secretary has been criticised by the SNP and climate activists after she chose not to use scheduled business-class flights and opted to take a plane dubbed the "Union jet" due to its Union Jack paint job.

The plane was used to transport Truss from London to Sydney on January 18, and back again five days later after fears of "security considerations" and concerns conversations could be overheard by other passengers on a commercial flight.

The Independent newspaper revealed that instead of travelling from London to Sydney on one of the daily Qantas departures, Truss insisted on flying the 22,000 miles to, from and within Australia on the private government Airbus A321. This type of plane holds 235 passengers on easyJet flights, but the government’s version is kitted out with a VIP interior, which includes lie-flat beds.

The Ministerial Code says only the Foreign Secretary could authorise such a flight.

A senior source from the aviation charter industry told The Independent that the cost of operating Truss’s trip would have been “at least £500k”.

READ MORE: Liz Truss spent £1400 on boozy lunch at Tory donor's restaurant

The most expensive business-class ticket for the Foreign Secretary’s entire itinerary on Qantas, which offers lie-flat beds on board the Boeing 787, would have been £7712.

SNP Environment spokesperson Deidre Brock MP said: "Yet again, Liz Truss has serious questions to answer over her grotesque misuse of taxpayers' money to fund her jet set lifestyle.

"Taxpayers will be appalled that the Tory Foreign Secretary has reportedly wasted over half a million pounds of our money on a luxury private jet trip – and emitted tonnes of unnecessary carbon emissions during a climate crisis."

She went on to reference Truss hosting a £3000 lunch with Joe Biden’s trade representative at an expensive private club owned by a Tory donor after overruling civil service advice to look for a cheaper venue.

"Ms Truss has a history of frittering away public funds. Only recently, she was criticised for splashing out over a thousand pounds on a single lunch, which saw her quaffing high-end wines in a Mayfair private members club owned by a Tory donor," Brock added.

"With a record like this, Lavish Liz will make a fitting successor to Boris Johnson.

"People in Scotland are looking on in horror at the arrogance, sleaze and corruption that has engulfed Westminster. The sooner Scotland becomes an independent country, and escapes the broken Westminster system, the better."

READ MORE: Speaker ‘not impressed’ with Tory's answer to claims about Liz Truss's £3000 lunch

Anna Hughes, director of the environmental charity Flight Free UK, said: ”Liz Truss has consistently demonstrated her lack of understanding about the action that we all need to take to address the climate crisis, and this is no exception.

“Flying to Australia and back on a chartered jet is shockingly privileged and an outrageous source of emissions in the current climate emergency.”

Truss's flight took off at noon on January 18 from London Stansted to Dubai. It departed two hours after the regular Qantas departure from Heathrow, which went nonstop to Darwin in Australia’s Northern Territory and continued to Sydney.

After a 90-minute refuelling stop in the UAE, the Foreign Secretary’s plane flew overnight to Kuala Lumpur. It landed Sydney shortly after midnight on January 20 – five hours behind the Qantas scheduled flight from London.

The plane sat on the ground at Sydney for two days while Truss met Australian politicians, before flying across to Adelaide where Truss gave a speech.

Data recorded by Flightradar24 shows Truss's Airbus left Adelaide at 10.50pm the same day. It refuelled at both Kuala Lumpur and Dubai before arriving back at Stansted airport 1.20pm on January 23.

The commercial alternative was a domestic flight the same evening from Adelaide to Darwin, connecting with the Qantas nonstop to Heathrow – arriving nine hours before the government aircraft.

The Foreign Office would not say how many officials were on board, but the Ministerial Code says delegations should be kept “as small as reasonably possible”. At least two sets of flight crew were required for the trip.

A Foreign Office spokesperson said: “It’s necessary for the Foreign Secretary to travel abroad to pursue UK interests around security, trade and technology, as she did during this visit to Australia.

“This trip used government transport and was fully within rules set out in the Ministerial Code.”