Two-thirds of enterprise wireless networks across the UK and Ireland are easily open to hackers.
Newell & Budge Security checked 2,000 wireless networks over the past year and found that 62% were wide open to attack.
The firm tested networks in Manchester, Birmingham, Bristol, Edinburgh, Glasgow, Belfast and Dublin, and found there had been no improvement in installed security since its last survey in 2003.
The worrying lack of security comes as the adoption of wireless Lans stars to take off.
Analyst Infonetics Research said worldwide wireless Lan hardware sales reached £424m in the second quarter of 2004. It added that this figure will rise by 9% in the second quarter of 2005.
Using a laptop PC with a wireless network card and running freely available hacking software, rogue users can detect poorly installed or turned off security on enterprise networks and public wireless "hotspots".
Newell & Budge found that up to 80% of networks in some areas were unencrypted and up to 70% were broadcasting Service Set Identifier (SSID) information, giving away corporate network information to would be hackers. SSID can be easily switched off by the IT department.