David Cameron has come under fire for using an Angolan plane on his high-profile trade mission to south-east Asia.
The Prime Minister pledged to "fly the flag" for British business as he arrived in Japan for the first stop of his tour.
In talks with counterpart Yoshihiko Noda, he lobbied for British companies to be given access to the country's previously closed defence market.
Cameron visits Japan
He also pushed the UK nuclear industry's case to help with the clean-up after last year's catastrophic earthquake.
Meanwhile, Japanese car giant Nissan announced that its Sunderland plant would be producing a new hatchback model from 2014 - creating hundreds of jobs. The governments have also signed an agreement paving the way for Japan to invest billions of pounds into British infrastructure projects.
But Labour questioned why the premier and representatives from more than 35 firms were travelling on a privately chartered plane from Angola.
Downing Street indicated that British Airways had been unable to provide a jumbo jet due to the Easter holidays, and other carriers such as BMI were also booked up.
Instead they opted to use operator Atlas - better known for moving cargo by air - who sourced a SonAir plane apparently owned by an oil company based in the African state.
The only patriotic marking visible on the craft is a Union Jack border around the door where Mr Cameron exits.
A source said: "We always approach British carriers first, but because we were travelling during a busy holiday season, they were unable to provide an aircraft for a five-day trip."