Nearly half of home wi-fi networks can be hacked in less than five seconds, according to a study carried out across UK cities.
In the ''ethical hacking'' experiment, researchers spent half an hour in each city using freely available software to use as many unsecured wireless connections as possible.
Almost a quarter of the private networks (9,249) had no password, despite 82% of Britons believing their network is secure.
But the study found even password-protected networks were not secure, with hackers able to breach a typical password in seconds.
The hackers were also able to ''harvest'' usernames and passwords at a rate of more than 350 an hour when sitting in town-centre coffee shops and restaurants.
CPP identity fraud expert Michael Lynch said: ''This report is a real eye-opener in highlighting how many of us have a cavalier attitude to wi-fi use, despite the very real dangers posed by unauthorised use.
''We urge all wi-fi users to remember that any information they volunteer through public networks can easily be visible to hackers. It's vital they remain vigilant, ensure their networks are secure and regularly monitor their credit reports and bank statements for unsolicited activity.''
Jason Hart, the hacker who carried out the experiments and the senior vice president of digital identity company CryptoCard, said: ''When people think of hackers they tend to think of highly organised criminal gangs using sophisticated techniques to crack networks.
''However, as this experiment demonstrates, all a hacker requires is a laptop computer and widely available software to target their victims.
''With the growth in the number of smartphones and wireless networks it has become far easier for hackers to crack usernames and passwords, allowing them access to emails, social networks, and online banking sites and even to assume the online identity of their victim.
You know that online banking that is so convenient for you? How convenient would it be if someone took less then 5 seconds to hack into your account?