Shielded enclosures, tempest equipment, shielded chambers

Another way of making sure you are not being bugged is to use a shielded tent, which prevents radio waves entering or leaving.

Mobile phone calls are impossible from inside the tent, but no-one will be able to listen to your conversations using bugs or radio wave listening devices.

It will also prevent anyone intercepting radio emissions from computers, preventing them from seeing what you have on screen.

This shielded tent will stop you doing anything other than having a conversation. It is a very crude, but very secure, way of talking," says Michael Marks, specialized in these devices.

A more sophisticated - and expensive - method is to build a "clean room", of the type used by the military, to shield radio waves and electromagnetic signals.