It has been one of the most annoying things about the internet for years: the little security check box that asks you to type out a barely legible phrase to prove you're not a robot.

But the CAPTCHA (Completely Automated Public Turing Test to tell Computers and Humans Apart) could be on its way out after Google invented an invisible version that discreetly monitors browsing patterns to determine whether a website visitor is a human or bot.

CAPTCHAs have been a necessary evil for internet users for years. Many websites require visitors to pass a test confirming they're human to prevent cyber attacks in which bots overload a website with traffic. Others such as Ticketmaster have used it to prevent ticket tout software.

But the CAPTCHA's original form, was a bane of many internet users, forcing them to squint and hazard a guess at a distorted jumble of letters to prove their humanity.