The makeover of the RAF aircraft used by the Prime Minister for official travel is expected to cost £900,000, Number 10 has revealed.
The Voyager aircraft, currently grey, is undergoing a red, white and blue Union Jack-style transformation in Cambridgeshire.
Downing Street said the "national branding" represents "value for money" amid criticism from some MPs.
Boris Johnson's official spokesman said the aircraft will retain its air-to-air refuelling capability and will be able to continue its "vital military role".
The spokesman said: "At every stage we’ve worked to ensure value for money for the taxpayer and all of the work has been undertaken in the UK, directly benefitting British suppliers.
"It will have national branding and that will mean that it will be in line with many other leaders' planes."
In response to the news, acting Lib Dem Leader Sir Ed Davey tweeted: "The drug dexamethasone, that can potentially save the lives of people with coronavirus, costs £5 per patient.
"Boris Johnson could have bought 180,000 doses of that, but instead he’s painting a flag on a plane.”
The Lib Dems' defence spokesperson, Jamie Stone MP, described it as an "absurd waste".
While Labour MP Emma Hardy said: “For goodness sake. Please can we have a grown up as a Prime Minister instead of a child.”
In 2018, then as foreign secretary, Mr Johnson said he would like to have a "Brexit jet".
He made the comments after questioning the availability of the RAF Voyager, which is also used by members of the Royal Family.
Mr Johnson was also quoted as asking: "Why does it have to be grey?".
The Voyager began transporting VIPs, including other Government ministers, in 2016 following a £10 million refit.
It was announced at the time that the 58-seater transport aircraft would continue to conduct air-to-air refuelling missions for the RAF when not in use and that it had retained its military livery.
Cover image: The RAF Voyager will be transformed from its current grey colour (Picture: PA).