Captcha systems to stop automated postings have been 'completely broken' by spammers, experts say. So what's the alternative? Tim Anderson reports.

Captcha is the bane of the internet," says Matt Mullenweg, who runs the massively popular blogging site Wordpress.com. "I can't figure them out myself half the time!"

He is referring to those squiggly, distorted images commonly seen when registering for internet services such as free email accounts or blogging sites. The user has to type the letters in the image before proceeding. Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart. The idea is that humans can read the letters, but computers cannot, thus preventing automated scripts from registering.