Wireless networks can be found all over London
A comprehensive seven-month audit found that 92% of the 5,000 wireless networks in the capital had not taken basic steps to protect themselves against casual attacks.
The survey, sponsored by the International Chamber of Commerce, used some novel software tools that could detect the networks other systems missed.
Many of the networks readily handed out internet connections to anyone that connected to them and almost all passed around confidential information in an easy to interpret form.
Site seizing
The survey shows just how popular wireless networks have become in only a few short years, and the risks companies are taking with sensitive information.
"It's the old story where you have convenience that has taken over prudence," said Pottengal Mukundan, director of the ICC's crime-fighting division.
Previous audits of the wireless networks in London have tended to concentrate on one area, such as Docklands or the City, and the networks found have numbered in the tens rather than hundreds.
But Simon Gunning, of technology security firm Digilog, has found that the capital is home to more than 5,000 wireless networks that are being used in offices, government buildings, prisons, police stations and even at the Palace of Westminster.
"I really didn't expect to find so many," he said.
He found them thanks to a software tool, freely available on the internet, which can spot wireless network access points that try to hide by turning off their ability to broadcast their presence.
Mr Gunning carried out the survey by driving around London in a car carrying a laptop fitted with the network-spotting software.
Networks exposed
Although wireless networks do have some basic built-in security features, the vast majority of networks found during this latest survey had not turned them on.
Even the few that had turned on the basic encryption system were using default settings, making it easy for an attacker to guess the key needed to unscramble data.
As a result, anyone gathering up packets of data from these networks would be able to read the text within them easily, said Mr Gunning.
Those not interested in stealing data could piggyback on the fast net connection linked to the wireless network.
Now, "war-driving", as it has become known, is a popular pastime with many curious computer enthusiasts.
Mr Gunning said many people were experimenting with long-range antennae that allow them to pick up signals from many kilometres away.
Security company i-sec has shown how an antenna made from an old can of Pringles crisps can help pinpoint networks.