Online images of child abuse will be flagged up as blacklisted to internet users in the latest bid to crack down on the problem.

BT has announced that any of its customers attempting to access web pages on the Internet Watch Foundation's list of identified images of child sexual abuse will now see a message telling them the site is blocked and the reason why.

Under the current system, the site is blocked but internet users only see an "Error 404" message.

The move comes amid growing concern that internet companies need to do more to tackle online child abuse.

BT's new message is believed to be the first of its kind in the UK.

It reads: "Access has been denied by your internet service provider because this page may contain indecent images of children as identified by the Internet Watch Foundation. If you think this page has been blocked in error please contact"

The firm began blocking illegal images of child abuse almost 10 years ago, and is believed to have been the first internet service provider to do so.

Online child abuse has become a high-profile issue in recent months, in the wake of a number of child murder cases with pornography connections.

April Jones killer Mark Bridger and Tia Sharp murderer Stuart Hazell were both found to have accessed child and violent pornography and some experts argue there is a clear link between their obsessions and their actions.

Speaking at a Westminster eForum on protecting children online, Tory MP Claire Perry, Prime Minister David Cameron's adviser on the sexualisation of childhood, said: "We've had a couple of appalling child murders. We always knew that it was going to happen – that there would be, whether it was causality or correlation, that there would be information or images found on the computers of those who went on to rape, abuse and murder children."

Ms Perry and Mr Cameron are next week due to meet with Google and other internet firms to discuss what more can be done to tackle online child abuse.

Research by the Child Exploitation and Online Protection Centre has suggested more than half of those who view child abuse images go on to commit abuse themselves.

Ms Perry also told the forum there was a big issue around children viewing "unpalatable content. "When we've got to a situation now where the average age, we're told, of a child first seeing pornographic material on the internet is 11-and-a-half years old, and 88% of porn involves violence against women – often extreme, depraved violence – you start to wonder what that's doing to the brains of 10 and 11-year-olds."

Ms Perry added that by the end of the year there would be filters that can protect every device in a home with one security setting.