The $300 billion Joint Strike Fighter project in the US, in which Britain's BAE Systems is a major partner, has been infiltrated by hackers.

BAE is working with Lockheed Martin, the lead contractor, and Northrop Grumman on what is the world's most expensive military programme.

But the Wall Street Journal reported today that hackers thought to be from China have broken into secret elements of the project, siphoning off several terabytes of data related to the design and electronics systems.

Such information could make it easier to defend against the stealth fighter planes. The incursion is the latest in a wave of attacks on sensitive US government data. Recent months have seen breaches by foreign spies of the air force's air traffic control system and the country's electrical distribution system.

Sources told the Journal that while spies downloaded a lot of information about the Joint Strike Fighter they could not get access to the most sensitive material, stored on computers not connected to the Internet.

The Chinese embassy said China “opposes and forbids all forms of cyber crimes.”