Facebook's rulebook on what images and videos should be allowed on its social network and which should be deleted has been revealed.

The secret documents, leaked to the Guardian, outline how its 4,500 moderators should police the millions of items posted to the site a day.

They shine a light on the guidelines that direct the company's secretive and often controversial censorship of posts.

Here are some of the posts Facebook has controversially banned.

Breastfeeding

Facebook has faced a seemingly unending struggle over how to moderate breastfeeding images over the past decade. The social network has had to reinstate countless images of mothers feeding their babies after mistakenly banning them for showing "obscene content" or "offensive nudity".

Back in 2008, thousands of people across the world protested having their breastfeeding photos removed from Facebook. They picketed the company's California headquarters, launched a petition called "Hey Facebook, breastfeeding is not obscene" and more than 11,000 people staged a "nurse-in".