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.