Facebook and Instagram have banned the private sale of firearms by unauthorised dealers.

Licensed firearm retailers will still be able to advertise on the site, but the updated policy will forbid individual users from conducting peer-to-peer sales of guns.

Facebook, which acquired Instagram in 2012, already prohibits the private sale of prescription drugs and marijuana.

The new rules can be read on Facebook's Community Standards page, under the Regulated Goods category.

Facebook already prohibited private firearms sellers from advertising "no background check required" sales, or offering transactions across US state lines.

“Over the last two years, more and more people have been using Facebook to discover products and to buy and sell things to one another,” Monika Bickert, Facebook’s head of product policy, said in a statement.

“We are continuing to develop, test, and launch new products to make this experience even better for people and are updating our regulated goods policies to reflect this evolution.”