BAE Systems said in a statement Tuesday that it had carried out full critical fuel testing on the short take off and vertical landing variant of the F-35 "five days ahead of its schedule proving the maturity of both the aircraft and program."

"The important fuel checks of the design and build of the aircraft was led by BAE Systems and involved constant testing, 24 hours a day for 13 days, which checked the calibration of the aircraft's fuel gauging systems. Testing took place at Lockheed Martin's Fort Worth plant in Texas where the first STOVL aircraft, BF-1, is currently undergoing a series of tests in preparation of first flight later this year," the company said.

Tom Fillingham, F-35 Lightning II managing director at BAE Systems, said: "It was a milestone moment in the program. This is the first STOVL we've tested, and we wanted to make sure the designs worked and check that the fuel systems had been put together properly."

BAE Systems said its team of engineers on the project were "based at Warton, Lancashire, and travel led out to Fort Worth to undertake the program of testing."

"Following the fuel testing the aircraft is undergoing structural coupling and ground vibration testing, as well as completing hydraulic testing of the doors and landing gear," the company said.

BAE Systems said it was "responsible for the design and delivery of the aft fuselage and empennage for each of the three F-35 variants, as well as key areas of the vehicle and mission systems, in particular the fuel system, crew escape, life support system and prognostics health management integration. The company also has significant work share in autonomic logistics, primarily on the support system side, and is involved in the Integrated Test Force, including the systems flight test and mission systems."

BAE Systems said it was also providing "significant elements of the program including electronic warfare, advanced low observable apertures, advanced countermeasure systems, vehicle management computer and active inceptor systems."