Slack, the workplace messaging app, has raised $427m (£332m) in a deal that values the San Francisco-based company at $7.1bn.

Founded in 2009, Slack has grown into a popular productivity app, used as a messaging and online work tool by companies. It aims to replace the need for internal work emails.

Slack allows workers to send messages, share documents and collaborate on projects using instant messaging. The group has over 1 million daily users and around 70,000 paying "teams" who subscribe to the service.

The start-up has now raised a total of $841m with the latest funding round led by Dragoneer Investment Group and General Atlantic.

In just four years, the company has grown to be one of the most highly valued "productivity" apps, competing against technology giants such as Google's Gmail and Hangouts work products, and Microsoft's new Teams workplace platform.

The company was founded in Vancouver by Canadian entrepreneur Stewart Butterfield, launching its current Slack product in 2014.