The Royal Air Force’s (RAF) Voyager aircraft have been undergoing trials ahead of the first nine planes – from a £10.5 billion fleet of 14 – being due to enter service in the coming months.
The leak is being worked on and the overall programme is not expected to be delayed.
The planes are also to be used as transports and medical flights and crews are training in these roles. Training on air-to-air refuelling is expected to begin once the fault is corrected.
A Ministry of Defence spokeswoman said: “Voyager is due to enter service with the RAF in its transport and aeromedical roles in the coming months.”
The dual role air-to-air tanker and transport plane has a 60-metre wingspan and is nearly 60 metres long.
The RAF has bought 14 of the aircraft under a 27-year £10.5 billion private finance initiative contract with the AirTanker consortium.