Contractors are the hole in the Pentagon's polo security -

Former national security officials have claimed that top Pentagon contractors have been handing over secrets to hackers for years.

Joel Brenner, the National Counterintelligence executive from 2006 to 2009, said most if not all of the big defense contractors' networks had been hacked.

The sort of break-in attempts at Lockheed-Martin have been happening since the late 1990s with the main threats coming from Russia, China and Iran, he told Reuters.

The spooks are after weapons systems and R&D.

Lockheed Martin said it had become "a frequent target of adversaries from around the world" probably because it builds F-16, F-22 and F-35 fighter jets, the Aegis naval combat system, and THAAD missile defence systems.In 2009 hackers broke into Lockheed's $380 billion-plus F-35 fighter project.

However it is unlikely to be the only contractor which has been troubled by hackers. Boeing, Northrop Grumman, General Dynamics, BAE Systems and Raytheon are not saying if their networks had been penetrated.

But Anup Ghosh, a former senior scientist at the Pentagon's Defense Advanced Research Projects Agency, or DARPA, said there had been a string of hacks into networks of US defence contractors, security companies and US government labs since the start of this year.

Ghosh said that the problem is that defence contractors failed to innovate information security and were fighting today's battles with cold-war era defences.