Harry and Meghan spent nearly £250,000 of public money on flights just months before quitting the royal family.
New accounts show the travel costs for the Duke and Duchess of Sussex's tour to southern Africa reached a staggering £246,000.
It has been branded a "disgraceful abuse of public money", but representatives have defended the expenditure.
Harry and Meghan took their baby son Archie, then four months old, to South Africa in September last year on his first royal overseas trip.
Harry then travelled solo to Angola, Malawi and Botswana.
The flights came to nearly a quarter of a million pounds and were the most expensive royal journey of 2019-2020, according to the royal financial records.
Action group Republic, which campaigns for an elected head of state, said there was no justification at a time when public sector jobs were being lost and services cut.
The trip became controversial after Meghan launched legal action against a newspaper and Harry delivered a scathing attack on the tabloid press while the tour was still under way.
A senior royal source insisted the couple - who recently netted a Netflix deal worth an estimated £112m - are under no obligation to pay money back for the trip.
The source stressed it was a key visit approved by the Foreign Office and helped to highlight the work of numerous charities.
"The Duke and Duchess of Sussex undertook over 20 engagements, bringing attention to a number of worthwhile causes, in particular, raising awareness of the work and the legacy of the Halo Trust," the source said.
"So, the visit, as an official visit funded by the Government, fulfilled the objectives that were set out for it and so therefore there would be no requirement or obligation on the Duke and Duchess of Sussex to make any payments in relation to that official visit."
Other costly overseas trips by members of the royal family included a two-day visit by the Prince of Wales to Oman to pay his condolences following the death of Sultan Qaboos bin Said, with the charter flight costing £210,345.
The source said the trip was taken at very short notice, and there were no commercial flights available.
The Duke of York took a charter flight to Northern Ireland to attend the Royal Portrush Golf Club's Open championship, which cost £15,848.
The royal source defended Andrew's use of a charter instead of a scheduled flight, saying: "In this particular case, we concluded that actually, the use of charter was the only way to get him to complete his engagements to fit in with his other programmes."
Andrew - once dubbed Air Miles Andy for his lavish use of helicopters and other aircraft at public expense - has since stepped down from royal duties over his former friendship with convicted paedophile Jeffrey Epstein.
The Princess Royal flew by charter to Rome to watch the Italy v Scotland Six Nations rugby match at a cost of £16,440.
For a trip to Pakistan, the Duke and Duchess of Cambridge's travel costs were £117,116.
The total cost of official travel for the royal family, met by the taxpayer, was £5.3 million in the last financial year - up 15% or £700,000 from £4.6 million in 2018/2019.
Buckingham Palace's Sovereign Grant financial report revealed the Sussexes' southern Africa travel costs came to £245,643, including the scheduled flights to and from South Africa, Harry's charter flights for his solo trips and scheduled flights for a staff planning visit.
Graham Smith, of Republic said: "At a time when public sector jobs are being lost and services cut, these figures represent a disgraceful abuse of public money by the royals.
"There is no justification for spending a quarter of a million pounds on a trip to Africa or more than £200,000 to fly to Oman.
"And they're all at. Thousands of pounds spent on junkets for Anne and Andrew, so they can attend sporting events. This is not public service, it's daylight robbery."
He added: "A 15% increase in travel costs when hospitals can't deliver the very best care to every person in need, when teachers are struggling to pay for the necessary books and equipment and the police are stretched to breaking point, is scandalous."
During the Sussexes' 10-day tour, Meghan gave a speech to young women in a township, and also tackled the issue of gender-based violence in South Africa, while Archie met Archbishop Desmond Tutu.
Harry brought back memories of the visit of his mother Diana, Princess of Wales to Angola, as he walked through a minefield in Dirico to see the work of a landmine clearance charity.
But the trip, carried out at the request of the Government, was overshadowed at the end when the duchess announced a privacy lawsuit against the Mail on Sunday over a letter she wrote to her father.
While still on the tour, Harry launched a scathing attack on the British tabloid press, accusing them of a "ruthless campaign" against his wife, adding: "I lost my mother and now I watch my wife falling victim to the same powerful forces."
He then filed his own legal proceedings against News Group Newspapers and Reach plc over alleged historical phone hacking.
Harry and Meghan also filmed an ITV documentary while on the trip in which they spoke of their struggles as royals.