More than half of Internet users have deleted cookies, seriously undermining website operators' ability to measure consumer behavior on their sites, a market research firm said Monday.

Cookies are small files often uploaded to people's computers as they visit websites run by retailers, entertainment companies, newspapers and other businesses. The applications are often used to track visitors' behavior on a site in order to build improvements later, or to offer visitors products or services they may be interested in based on previous visits, a process called personalization. In addition, cookie-gathered information is often pivotal for advertising campaigns and email marketing.

"Cookies, 99 out of a 100 times, are not an invasion of a consumer's privacy or security," Eric T. Peterson, analyst for JupiterResearch, a division of Jupitermedia Corp., said. "They're just harmless little text files."

Nevertheless, 58 percent of Internet users have deleted the tiny files, essentially making many consumers anonymous during site visits, and crippling website operators' ability to gather information, JupiterResearch found through surveys this year of more than 4,600 online consumers. In addition, 39 percent of consumers are deleting cookies from their primary computer monthly.

The reason for these Draconian measures is fear. Consumers are constantly reminded about the risks on the Internet posed by spyware, phishers and viruses, so deleting cookies makes them feel more secure, even though it's unlikely to make them safer, Peterson said.

"It makes sense that consumers are going a little over the top to try and find things that they can do to protect themselves online," Peterson said.

Fully 52 percent of consumers who use the web say they have a strong interest in stories and articles about Internet security and privacy, JupiterResearch found. The high level of concern contributes to 38 percent of consumers believing cookies are an invasion of security and privacy, while 44 percent are convinced that deleting or blocking cookies protects them.

Peterson said the problems caused by cookie deletion are going to get worse for businesses.

"They shouldn't put their heads in the sand and hope the problem goes away," Peterson said.

Instead, website operators need to look for technologies other than cookies to gather information. For example, Flash, a website-development technology from Macromedia Inc., can track consumers each time they visit a site.

For those companies that want to continue using cookies, they should consider telling visitors upfront that cookies are used and clearly explain why.

"It would also behoove them to ask the consumer first, and get permission," Peterson said.

Anti-spyware makers could also help by not deleting cookies out of hand, but leaving those that present no threat to consumers, Peterson said. It's unlikely, however, that anti-spyware vendors would do that, since consumers want all cookies deleted.