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 hundreds of developers outside the company full access to users' inboxes.

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

Google requires users to approve access to the apps that they install on their accounts and details the data that it will gather, although it does not specify that staff, rather than machines, will be able to scan individuals' inboxes.

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.

Hundreds of outside software developers were granted access to private emails through the terms and conditions of the user agreement. There is no suggestion of misuse of data by Google or its third-party providers.

Google declined to comment.

Irish Independent