New Defence Secretary Gavin Williamson has yet to get to grips with the complexities of the defence budget, Chancellor Philip Hammond has suggested.
Appearing before the Commons Treasury Committee, Mr Hammond denied he was cutting funding to the armed forces and indicated he would be sympathetic to a request for extra cash.
His intervention comes amid signs of an extraordinary briefing war between the two ministers over proposed cuts to the military to plug a black hole in the Ministry of Defence (MoD) finances.
In the latest twist, The Times reported that Mr Hammond had been banned from using a fleet of RAF jets and helicopters until the Treasury settled a bill for his official travel with the MoD.
The MoD was said to have told officials not to accept any more bookings from the Chancellor until the six-figure sum for flights with No 32 (The Royal Squadron) had been paid.
Downing Street was drawn into the row with the Prime Minister's spokesman acknowledging that there was an "administrative discussion" under way between the two departments.
Mr Hammond meanwhile suggested he would welcome a chance to discuss the defence budget with Mr Williamson - formerly the chief whip - once he properly understood the figures.
"There is no greater champion of defence than me. I was defence secretary for almost three years. I am a huge advocate for our armed forces," he said.
"There is no question of the defence budget being cut. The defence budget is being increased but I recognise also the defence is facing some pressures, particularly around currency movement. A lot of defence procurement is denominated in US dollars.
"I expect that once he has had a chance to understand the situation in the Ministry of Defence and to get his head around the defence budget, the new Defence Secretary will be wanting to come and talk to me and he will find no one more sympathetic to the challenges of defence than me."
It is understood that Mr Hammond used BAe 146 jets and A109 helicopters to fly to more than 20 destinations this year on official business, including Manchester, Leeds, Wolverhampton, Dudley, Aberdeen, Paris, Brussels and Frankfurt.
Prime Minister Theresa May's official spokesman told reporters: "My understanding is the Chancellor used a Government jet yesterday.
"I gather there is an administrative discussion going on. This matter is being resolved."
A Downing Street source said the issue had not been raised with Number 10, but was "being sorted out between the two departments".
A spokesman for Labour leader Jeremy Corbyn said: "The Government's problems and the Cabinet's problems are multifarious.
"This is a Government in a shambles on both the most important issues facing the country and the more minor ones, and I would suggest that this is one of those.
"I'm sure we would be in favour of the Treasury paying its bill."