Leaked documents reportedly show that Chinese spies hacked large amounts of data relating to the design of the F-35 Joint Strike Fighter jet.
Media reports suggest the alleged cyber theft is revealed in documents leaked to the German magazine Der Spiegel by former US security analyst Edward Snowden.
The F-35 is a stealth bomber developed by the US-based defence firm Lockheed Martin. The Royal Air Force is among the major customers for the jet.
The Sydney Morning Herald cites one document as saying that the theft amounts to "many terabytes of data" including information about the F-35's radar systems, engines and method for cooling exhaust gases.