The plane used by Boris Johnson and members of the royal family for international travel is being repainted in the colours of the Union flag to “better represent” the UK abroad.
The red, white and blue “rebranding” will cost about £900,000, No 10 said.
Downing Street said it represented “value for money” and that all of the work was being done in the UK.
But opposition parties were critical, saying the money would be better spent on helping the victims of coronavirus.
No 10 said the aircraft was currently in Cambridgeshire for pre-planned repainting in “national branding”.
Asked about the plane at the daily coronavirus press briefing, Culture Secretary Oliver Dowden said “we have always spent money on promoting the UK round the world” and added that the work on RAF Voyager was part of that effort.
The RAF Voyager is used by the prime minister, other ministers and senior members of the Royal Family for official engagements.