Using a free and unprotected wireless Internet network? Beware, your computer system can be hacked in less than five seconds.

A new British study has found that nearly half of homes in Britain with wireless Internet networks (wi-fi) are at risk of being hacked in just seconds.

It found that a quarter of wi-fi connections in the UK have no password attached, making them easily accessible to cyber criminals who can spy on their activities, steal their identity and banking details to raid their accounts.

The hackers can also use the wi-fi access to tap into illegal pornography or upload and download stolen music and movies without being traced, the Daily Mail reported.

According to the study, commissioned by Britain's card protection and insurance firm CPP, wi-fi networks in millions of homes can be hacked in just few seconds.

The researchers, who carried out an ethical hacking experiment in six cities across the country by using freely available software, found almost 40,000 home wi-fi networks at high risk.

Separately, they found nearly a fifth of wireless users were using free wi-fi networks offered by coffee shops and other businesses.

They also found that hackers were able to "harvest" usernames and passwords from unsuspecting people at a rate of more than 350 an hour, sitting in restaurants or coffee shops.

CPP fraud expert Michael Lynch said this report is a real eye-opener in highlighting how most of people have a cavalier attitude towards wi-fi use.

"We urge all wi-fi users to remember that any information they volunteer through public networks can easily be visible to hackers," he said.