More than half of computer users have illegally logged on to someone else’s wi-fi connection yet only 11 people have been arrested for the crime, an investigation by The Times has found.

“Wi-fi tapping” or “piggybacking” has boomed in the past few years as hackers take advantage of unsecured computers to access the internet without paying for it.

Police regard it as a serious offence because intruders can download pornographic materials and illegal images without being caught. Only the legitimate holder of the wi-fi account is likely to be tracked down.

Officers are also worried that criminals can use unsecured wireless connections to steal personal details such as passwords and credit card numbers and use them to commit identity theft.

At the very least, those who fall prey to wi-fi tapping will find that the speed of their internet connection slows down and may grind to a halt.

Anyone found guilty of using someone else’s broadband connection without permission faces a maximum fine of £1,000 and up to five years in jail.

But a survey of the police forces in England and Wales by The Times has found that officers have arrested just 11 people since the activity became an offence. It breaches both the Communications Act 2003 and the Computer Misuse Act 1990.

Arrests have been made by forces in Devon & Cornwall, Essex, Norfolk, Lincolnshire, West Mercia, London and Northamptonshire for the crime, also known as “freeloading”.

A spokesman for the Serious Organised Crime Agency said: “This is something that is fairly difficult to trace. But there is a certain responsibility that the individual has to assume in the fight against this. For example, they should turn on their security measures so they don’t leave themselves open to a blatant attack.”

In July 2007 a 24-year-old man was arrested in Newquay, Cornwall, after being caught using his laptop outside a home in a residential street. Offenders have also used the connections of internet cafés, offices and, in one case in Essex, a school.

Although wi-fi routers, which allow a homeowner or business to access broadband, have security functions, nearly eight million people in Britain have left their internet access unsecured, according to GetSafeOnline. org, a government-backed organisation. Tony Neate, managing director of GetSafeOnline, said: “I think unsecured internet is a big problem.

Consider, for instance, that computers and laptops automatically search for wireless networks.

“If you do something you think you shouldn’t have done and the police get involved, they trace it back to the person who has the internet access, not to the person who has done the offence. As the owner of the connection, you would have a job to persuade the police it wasn’t you.”

Sophos, a global internet security firm, carried out a survey on behalf of The Times. It discovered that 54 per cent of computer users have secretly used someone else’s wireless broadband connection without paying for it.

Graham Cluley, senior technology consultant at Sophos, said: “I think that a lot of the general public probably view this as a ‘victimless’ crime, and are prepared to hop on board someone else’s wi-fi connection, even when they have never met the person or visited the company supplying it.

“The message is loud and clear – properly secure your wireless access to prevent freeloaders and hackers from abusing your internet connection. You wouldn’t leave the doors wide open in your house, so why allow it with your internet access?”

Illegal wi-fi tapping first emerged in the US when Benjamin Smith III was arrested in Florida in 2005. The first case in Britain came later that year when Gregory Straszkiewicz, 24, was fined £500 and given a 12-month conditional discharge after using a laptop from his car in West London.

How it works – and how to prevent it

Isn’t this a victimless crime?

No. A crime is perpetrated against the person who pays for the internet connection. The Communications Act 2003 says a “person who (a) dishonestly obtains an electronic communications service, and (b) does so with intent to avoid payment of a charge applicable to the provision of that service, is guilty of an offence”.

OK, but is anyone really doing this?

Yes. Sophos, a global internet security firm, has found that 54 per cent of computer users have admitted to using someone else’s wi-fi without the owner’s permission. GetSafeOnline.org says that more than 7.8 million people in Britain have left their internet access unsecured.

So how does it work?

The hacker needs to be within about 50 metres of your computer. The hacker’s computer searches automatically for a wireless connection – and picks up yours.

Surely the hacker will be caught?

Not likely – it is very difficult to track. Plus, if the hacker has used your broadband to log on to an illegal site, this will be traced back to your wireless router. Then it is your job to persuade the police that you are innocent.

How do I guard against this?

Change your wireless router’s default password, which can be something as obvious as “password”, to something that hackers cannot guess. You can also configure your router to allow wireless access only to devices you know about. In addition, there will be an option on your wireless router to encode transmissions.