A change in the terms and conditions of Yahoo's free webmail service means the tech giant can snoop emails sent and received by users.

"By using the Services, you consent to allow Yahoo!'s automated systems to scan and analyse all incoming and outgoing communications content sent and received from your account (such as Mail and Messenger content including instant messages and SMS messages)," the terms and conditions now read.

"This includes Mail and Messenger content already stored in your account. Unless expressly stated otherwise, Yahoo! Mail customers will not be allowed to opt out of this feature."

Furthermore, Yahoo is making users of the webmail service responsible for alerting non-Yahoo users about the potential snooping.

"If you consent to this ATOS and communicate with non-Yahoo! users using the Services, you are responsible for notifying those users about this feature," the Ts&Cs state.

Yahoo said the move will help the tech giant ensure content - and, in the future, targeted ads - is relevant to its users, as well as helping to protect web users from spam and messages containing malware.

"As part of Yahoo! Mail Beta, we will provide personally relevant product features, content, advertising, spam and malware detection by analysing (via machine scanning) inbound and outbound emails. Some of these features and advertising will be based on our understanding of the content and meaning of users' emails," the company said.

Daniel Hamilton, director of Big Brother Watch, told Which? he was extremely disappointed that Yahoo had "opted to intrude on the privacy of their users in this way".

"These changes hand the company the power to snoop on and analyse vast amounts of personal data. Web users have a right not to see their personal communications trawled through in order to boost Yahoo!'s advertising revenue," he said.

"Yahoo! should abandon these changes before the crucial bond of trust between it and its users is damaged beyond repair."