According to the Wall Street Journal, this permission sometimes allows employees of third-party apps to read users' emails.

'Not asked permission'

While messages are typically processed by computer algorithms, the newspaper spoke to several companies where employees had read "thousands" of email messages.

Edison Software told the newspaper it had reviewed the emails of hundreds of users to build a new software feature.

Another firm - eDataSource Inc - said engineers had previously reviewed emails to improve its algorithms.

The companies said they had not asked users for specific permission to read their Gmail messages, because the practice was covered by their user agreements.

"You can spend weeks of your life reading terms and conditions," said Prof Alan Woodward from the University of Surrey.

"It might well be mentioned in there, but it's not what you would think of as reasonable, for a human being in a third-party company to be able to read your emails."

Google said only companies that had been vetted could access messages, and only if users had "explicitly granted permission to access email".

It pointed the BBC to its developer policies, which state: "There should be no surprises for Google users: hidden features, services, or actions that are inconsistent with the marketed purpose of your application may lead Google to suspend your ability to access Google API Services."

It said Gmail users could visit the Security Check-up page to see which apps they had linked to their account, and revoke any they no longer wanted to share data with.