Citizens wanting to join the UK's secret intelligence agencies should be careful about what information they put online and which social networks they access, according to IT security consultants NCC Group.

The company claimed that British intelligence agencies are concerned at the effect on the spy industry of social networking tools like Facebook.

NCC has warned individuals seeking a career as a spy not to participate in online networks and discussions. The advice was prompted by the wife of Sir John Sawers, the next head of MI6, posting family details and photographs on her Facebook page.

"When working undercover you obviously need to maintain a low profile at all times, and that means having no images in the public domain, or being associated with any person or organisation," said NCC chief executive Rob Cotton.

"When you have a Facebook profile you are immediately doing just the opposite, and furthermore, opening up a Pandora's box of online traceability that you can't ever truly close.

"This extends to close associates too. Do you know what your partner, child or friend is writing about your life?"

Cotton explained that the problem for national security staff is that, once their details are out on the web, it is very difficult to erase them, and the wrong details in the wrong hands could put lives at risk.