Nearly half of the UK's home wi-fi networks can be hacked in less than five seconds, according to a new study.
The study was carried out in six UK cities led by so-called 'ethical hacker' Jason Hart, vice-president of network security firm CRYPTOCard.
Hart's team carried out the research using freely available 'WarDriving' software designed to sniff out insecure wireless networks.
The research found that nearly 40,000 networks were at risk from hackers, leaving their users' personal data prey to identity fraudsters and other cybercriminals.
According to the study's findings, nearly 25 per cent of the private wireless networks found had no password protection at all, making them instantly accessible. Another quarter of networks employed only the older WEP type of encryption, which can be cracked in a matter of seconds.
The study also revealed how many users were willing to put their computers at risk by logging on to unprotected sites. Over the course of an hour, more than 200 users logged onto a fake wi-fi network set up by the researchers.
Over the course of the study, researchers were also able to 'harvest' more than 350 usernames and passwords an hour from unsuspecting users in town-centre coffee shops and restaurants.
Michael Lynch, identity fraud expert at life assistance company CPP, which commissioned the study, 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."
Earlier this month, Metropolitan Police commissioner Paul Stephenson warned of an acute lack of resources to target online crime at the force's Police Central e-Crime Unit (PCeU).
In an article written for The Telegraph, he admitted the unit could only deal with a tenth of known cybercriminals, with just 15 per cent of its 385 officers dedicated to terrorism, online fraud and identity theft.