The FBI is warning against the popular practice of using chalk marks to show the location of wireless networks.

The marks, or 'warchalks', are cropping up in cities and suburbs across the world. The FBI has told companies that, if they see the marks outside their offices, they should check the security of wireless networks and ensure that they remain closed to outsiders.

"If you notice these symbols at your place of business, it is likely that your network has been identified publicly," said the agency.

Although the warning is not an official advisory, the agent responsible for it said that it was information worth passing on.

He urged companies using a wireless network to ensure that it is secure and used only by authorised personnel.