Google allows employees at software companies to read millions of Gmail users' private messages, it has emerged. The popular email service, which has more than one billion users around the world, gave developers outside the company access to inboxes.

They were then able to read and scan private emails to target adverts.

Gmail allows its users to install additional apps that work with the online system. Some allow people to write emails in special fonts, or to make it easier to find images to send to others, while others make it easier for people to organise their emails into folders.

Millions of people are believed to have installed Gmail apps. However, installing them hands the app developers full access to users' inboxes. In some cases, staff at the companies were able to read users' private emails, according to the Wall Street Journal.

They include Return Path, a company that collects data for advertisers, and email organisation tool Edison Software. Users had to deliberately opt out from granting these apps access to their Gmail accounts to stop third party companies looking at their data.