Google announced Monday that it is shortening the lives of software "cookies" used to track users' online preferences.

In coming months Google will begin issuing cookies that automatically expire two years after a person visits the website provided they don't return, according to the US firm's global privacy counsel Peter Fleischer.

"We've concluded that it would be a good thing for privacy to significantly shorten the lifetime of our cookies as long as we could find a way to do so without artificially forcing users to re-enter their basic preferences at arbitrary points in time," Fleischer wrote in a Google blog post Monday.

Online privacy advocates expect Google's new "cookie policy" to change little since the two-year lifespan of tracking software renews with each visit so people must stop using Google for the entire period for the cookies to self-destruct.

Cookies previously installed on computers by Google are made to expire in 2038.

Google and other Internet firms put bits of code called "cookies" on users' computers to tailor services, for example determine whether a search for "WWF" should get World Wildlife Fund or Worldwide Wrestling Federation as a result.