Schools across the UK are warning parents that the online Momo "challenge" is being spliced into YouTube videos of Peppa Pig and Fornite.

Momo is a meme that targets young children on social media, encouraging them to add a contact on messaging platform WhatsApp or online games, and then sends them graphic images and messages.

Police have expressed concern about the meme, which it says is being used by cyber criminals to harvest information.

This week, schools began publishing warnings on their websites and social media platforms explaining that they had been contacted by concerned parents.

The schools added that clips featuring the “Momo” character, who is symbolised by a woman with bulging eyes, appear in the middle of videos of children’s cartoon programme Peppa Pig and computer game Fortnite on YouTube and YouTube Kids - a version of the service oriented towards children.

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On Tuesday, Haslingden Primary School, Rossendale, near Blackburn wrote in a post on Facebook: 'We have become increasingly aware of highly inappropriate videos circulating online and are being viewed by children across the school."

The school explained that one of the videos “starts innocently, like the start of a Peppa Pig episode” but quickly turns into “an altered version with violence and offensive language”.

It described another video clip that includes Momo's “warped white mask” that encourages children to do life-threatening tasks, from turning the gas on or taking tablets, without telling their parents.

“As you can imagine, this is highly distressing for the children to view. We encourage you to be vigilant when your child is using any device or watching any clips,” the post read.

A similar Facebook post by Newbridge Junior School, near Portsmouth read this morning: “Today we've heard from many concerned parents and pupils about the horrifying #Momo challenge which has reportedly been appearing in children's YouTube videos, causing panic and upset amongst young people. PLEASE CHECK WHAT YOUR CHILD IS LOOKING AT ONLINE.”

A post from Newbridge Junior School about the 'challenge' on 27 February 2019 ( Faceboook: @NewbridgeJuniorSchool )

National Online Safety (NOS), a group of online safety experts that provides advice for schools school staff, parents, and children has published a list of tips to help parents and carers learn more about Momo in order to protect their children.

NOS’ advice, posted on Twitter on Tuesday, includes telling children that Momo is not real, being present while they are online, talking to them regularly about their activity online, and setting up parental controls on devices at home.

Police have also issued warnings about the challenge.

Authorities in Northern Ireland have said: "Even basic open source research suggests that 'Momo' is run by hackers who are looking for personal info.

"Whatever or whoever is behind it, there is no disputing the content being sent is horrendous.

The police added that the “challenge” can appear on countless apps with a chat function.

They explained: "A 'curse contact' sends a number and tells you to contact them on Whatsapp. One video of such interaction in America I've seen shows an ominous sounding voice recording being sent to a child telling them to take a knife to their own throat.

"Another threatens family if a 'challenge' is not completed. It's chilling viewing. There are numerous variations and of course now imitators."

On Wednesday, YouTube said that content relating to “Momo” has not been found on YouTube Kids and that YouTube permits news stories and videos intended to raise awareness of and educate against the “challenge”.

A spokesman for YouTube said: “Contrary to press reports, we have not received any links to videos showing or promoting the Momo challenge on YouTube.

“Content of this kind would be in violation of our policies and removed immediately.”