Do you know what's on your kid's smartphone?

Children are being bullied using a popular app that allows people to send messages anonymously, schools have warned.

The Manchester Evening News reported that Tellonym lets its 8 million users share texts with each other in “the most honest place on the internet”.

It is promoted using the fact that it lets users “see what your friends think of you, answer anonymous questions and ask others the things you have never dared before”.

Two schools in Greater Manchester have put out a warning to parents, telling them the app can lead to bullying.

The message was sent out jointly by Urmston Grammar in Trafford and Abraham Moss in Manchester. It said some some of the bullying incidents at the schools had led to the police getting involved.

The statement said: “We have been made aware of the potential dangers associated with the use of the application ‘Tellonym’.

“The app is available as a free download on the Apple App Store and Google Play store and is not regulated. It is aimed at children aged 9-plus.

Some of the criticism has been disputed by the app's creators

“Tellonym advertises itself as ‘the most honest place on the internet. See what your friends think of you, answer anonymous questions and ask others the things you have never dared’.

“It allows anonymous postings, often inappropriate messages, comments and photographs which have caused upset and distress to young people.

“We are aware that some of these posts have required intervention from the police and other services.

“We are writing to make you aware that your son/daughter may be using this app and you may therefore wish to talk to them about e-safety and appropriate use of such apps.”

Screengrab of homepage of anonymous messaging app Tellonym

The app is completely free and once downloaded, users can receive questions or comments anonymously from people who know them.

It has already faced criticism in Germany, where it was first launched, over claims users can be sent sexually explicit messages and insults.

Maximilian Rellin of the developer Callosum Software told the German newspaper Der Spiegel: “Cyberbullying is unfortunately a widespread problem these days”.

Mr Rellin added that the terms of use of the app also prohibit messages that are ‘sexually explicit’ or ‘intended to offend anyone’.

But in a statement debating some of the claims being made by the school, Tellonym said it deleted many inappropriate posts once they were alerted to them.

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The app creators said in a statement: "The objection that Tellonym is 'not regulated' is rather baffling. To date, there is no governing regulatory body to which Tellonym can submit. Instead, we are held to high standards by Apple and, by default, Google.

"In the author's eyes, this may not be sufficient - and we’d be inclined to agree - so we hold ourselves to better standards.

"Strangely, the author gives the impression that Tellonym is a general multimedia messaging system. Happily it is not! Instead we have a uniquely terse, text-only format which users love and which, by design, prevents trolls and bullies. No threads, no commenting, no images or movies, no uploads of any sort. It’s a simplicity to be enjoyed and which we’re at pains to protect."