Warchalking, the technique of highlighting areas where wireless networks can be accessed freely, has been blasted as theft.

And the practitioners of warchalking are being slammed as bandwidth thieves in an advisory issued by mobile and wireless vendor Nokia.

Over the last few months, geeks have been drawing chalk symbols on walls and pavements in cities to mark points where signals from nearby office wireless networks can be tapped into to access the internet.

The initial hysteria was over security, when it emerged that warchalkers may also be freely browsing corporate networks and accessing private company information. Now Nokia has raised the stakes.

"Data privacy is at stake, and so is data integrity," the firm said. "But the little-talked-about issue of bandwidth-robbing by these warchalkers should not be ignored.

"While the warchalkers maintain they are not trying to hack networks, they are using a resource which they haven't paid for."

Sitting outside an office and using a company's wireless network to surf the web means that the perpetrator has established an IP address and is using bandwidth - reducing the bandwidth available to the company. "This is theft - plain and simple," said Nokia.

Another problem that has presented itself in recent weeks is that of 'warspamming'. Simply by logging into an unprotected wireless network and finding an open simple mail transfer protocol port, spammers can send their messages to 10 million names while remaining completely anonymous, as well as avoiding heavy bandwidth costs.

"If the connection is used often enough with data-hungry applications, the warchalkers could steal enough bandwidth to reduce the performance of the company's applications, resulting in a poor end-user experience and potentially even denial of service. For a business, this is unpalatable," said Nokia.