With three teenage boys in the house, Sally Shaw and her husband Simon, an Army officer, thought that they had taken every possible precaution with regard to internet safety.

The boys, Sally's stepsons, were banned from using devices in their room after 10pm, and the wifi in their four-bedroom home in Derby was switched off at night.

Little did they know that one of the boys - 14-year-old Matthew - was secretly switching it back on in order to watch porn.

'We first noticed his behaviour change when he got his iPhone,' says 41-year-old Sally. 'He was spending a lot of time upstairs, but we thought maybe he'd got a girlfriend.

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A survey for the BBC last year found that 60 per cent of young people were 14 years old or younger when they first saw porn online (file photo)

'One night, I found him in his room having a conversation with two school friends on the phone while they all watched the same porn on their iPads.

'He jumped up and tried to hide what he was doing, but what I'd seen was revolting. I couldn't believe it.'

Furious and upset, Sally and Simon, 43, grounded their son for a week and took away his phone and iPad.

After several frank conversations in which they imposed even stricter rules about internet usage, they hoped the problem was resolved. In fact, it was just the beginning of their nightmare.

'He would find ways to get around us,' says Sally, a full-time mother. 'He kept offering to hang out the washing in the garden and I thought he was being helpful.

It was only later that I discovered he was sitting at the bottom of the garden accessing the neighbours' wifi.

'We tried taking the phone away but he would just get the iPad from his younger sister. Or he would borrow one from a school friend. He managed to change the parental controls on our wifi so that only he knew the code.

'We discovered there was a core of about five boys involved; a kind of 'porn ring' who would watch these videos in tandem so they could see each other's reaction to it. I found it really disturbing.

'We didn't want to take his phone from him permanently, because it's a two-mile bike ride to school and we wanted him to be safe.'

This paradox is one that many loving parents find themselves wrestling with today.

With 58 per cent of mobile phones now having access to the internet, children are able to access pornography with alarming ease (file photo)

Latest statistics show that 81 per cent of 13 to 18-year-olds own a smartphone, while 43 per cent of 12 to 15-year-olds own a tablet such as an iPad.

With 58 per cent of mobile phones now having access to the internet, children are able to access pornography with alarming ease.

'Children can find sexual material fairly easily on the internet, whether as a result of curiosity or just by accident,' says Carolyn Bunting of Internet Matters, an organisation that educates parents about the risks their children might encounter online.

'This can be confusing and upsetting because pornography portrays unrealistic images of sex and relationships.'

Worryingly, in some cases youngsters are becoming fixated by porn.

A poll of nearly 700 children by the NSPCC last week revealed that nearly one in ten 12 to 13-year-olds is worried about having an addiction to porn, while more than one in ten have made or been part of a sexually explicit video.

The disturbing results also showed that one in five of those surveyed said they'd seen pornographic images that had shocked or upset them.

This is just the latest survey to concern child welfare experts. A survey for the BBC last year found that 60 per cent of young people were 14 years old or younger when they first saw porn online.

Latest statistics show that 81 per cent of 13 to 18-year-olds own a smartphone, while 43 per cent of 12 to 15-year-olds own a tablet such as an iPad (file photo)

Another report last year, by the charity ChildWise, revealed that the website Pornhub was named in the top five favourite sites by boys aged 11 to 16.

For Sally and Simon, things went from bad to worse.

'Matthew became increasingly moody,' Sally says. 'He didn't want to integrate in family life. He couldn't wait to get away from the table after dinner. He became very withdrawn.

'I tried to speak to the parents of the other boys involved but got nowhere. I was met with a lot of comments like: "Our son would never do that…"'

Matthew didn't speak to me for several days after I'd been to the school, but as far as we know the 'porn ring' ended there. It's a morbid fascination, and what's disturbing is that it's very explicit sex, without any romance around it. It gives children a distorted view of relationships Mother-of-three, Sally

In desperation, Sally went to the school and spoke to the headmaster, who called in Matthew and his friends. It emerged that the 'porn ring' had begun when one of the boys had accessed online porn via video-sharing website YouTube.

Sessions were subsequently arranged for the boys with the school counsellor, both individually and with their parents.

Sally says: 'Matthew didn't speak to me for several days after I'd been to the school, but as far as we know the 'porn ring' ended there. It's a morbid fascination, and what's disturbing is that it's very explicit sex, without any romance around it. It gives children a distorted view of relationships.

'The problem is that it's too easily accessible, and the temptation is there all the time.'

Laura Kay is another mother who was horrified to discover that, at the age of just ten, her son Nathan had been accessing porn. And this was despite the fact she'd put filters on all the devices in their home.

'I'm pretty tech-savvy and thought I'd done everything I could to stop Nathan accessing porn. So when I found him asleep upstairs, with his iPad open, and saw that he'd been looking at really hardcore stuff, I was devastated,' says Laura, 43, a social media manager who lives with Nathan, now 13, in Exeter.

'An older friend had shown him how to bypass the filters and then had directed him to this site. I was heartbroken. My little boy's innocence had been shattered.

'Next day, I confronted Nathan and he broke down in tears saying that he didn't want to look at it but the friend had made him. I was so angry.'

Since the incident, Laura says she has changed passwords and kept a very close eye on her son's online activity, but on occasions she has been stunned by what she has uncovered.

'I banned him from talking to two friends online last year because I couldn't believe the sexual language that they were all using.

'It's not just boys either. These friends were 11 and 12-year-old girls using the 'C' word all the time and saying things like: 'Do you want to bang me?'

'Where do they get this terminology from? I've met these girls in person and they are the most polite youngsters you'd wish to meet. You'd never dream that they could use words and phrases like that, but they're all doing it.'

Laura's problems didn't end there. Last year she found that Nathan — like 60 per cent of teens — had been asked for a sexual image of himself.

'I was called into Nathan's school because a girl had made a complaint about him asking her to send him topless pictures.

'In fact, the girl had been asking Nathan for photographs of his penis and he'd copied some photographs he found on the web and sent them to her.

'She'd also sent him cleavage shots but only complained when Nathan sent one of them to his friend, who then asked her to send him some, too.

'The Government really needs to take serious steps to stop this. We, as parents, have to start talking about it, and schools too.' The extent to which porn is affecting our children in the long term is something experts can't agree on, but psychologist Professor Geoffrey Beattie says young people may be more damaged than we realise.

He is most concerned about the potential for psychological harm caused by 'flashbulb memories'. 'Many of our experiences of everyday life are quickly forgotten,' he says, 'but there are things that we would like to forget, yet cannot, no matter how hard we try.

'There are images and events that stick in our brains and never seem to fade with time: the plane going into the Twin Towers, the image of the bus on 7/7, or the death of Diana.

'You remember the whole social context, such as where you were, who you were with, what was said and the expressions on the faces of others.

'These types of vivid memories are called flashbulb memories and are a central part of post traumatic stress disorder because they do not fade with time. But these are the sort of traumatic images that our children are seeing every day.'

There's another concern that the kind of imagery that youngsters are seeing will affect their relationships in the future.

Suzie Hayman, trustee of the charity Family Lives and author of How To Raise A Happy Teenager, says: 'The pornography that so many youngsters now view online empties sex and relationships of any emotional content.

'Sex becomes something perfunctory, an act devoid of love, respect or fun.

'Many young people we speak to say that porn can increase sexual bullying as sexual expectations can become unreasonable. Young people feel pressured to carry out degrading acts that replicate what has been shown on these films.

'Unfortunately, people who watch too much porn can find it hard to relate to others in the real world on an emotional level.'

So what is being done? The Government recently proposed plans for children aged 11 upwards to be taught about rape and sexual consent in schools. A depressing sign of the times, perhaps, but this would include discussion around what they have learnt from watching pornography.

Meanwhile, Culture Secretary Sajid Javid recently made a commitment to introduce age verification to websites that allow children unrestricted access to pornography.

While experts welcome the move, they warned it would be hard to implement in practice, so parents would still be the first line of defence.

'Parents can take positive steps to ensure their children only see age-appropriate content on the internet,' says Carolyn Bunting of web safety charity Internet Matters. 'This includes setting parental controls on the broadband and search engine, keeping an eye on their browser history and apps they have downloaded onto their phones.

'It's never too late to have a conversation with your child about online pornography.'

In fact, several apps are available that track what a youngster is looking at. New software called Mobile Force Field has recently been launched which switches off any apps that parents don't want their children to use and stops them from sending or receiving inappropriate selfies.

For some parents, however, it's already too late to protect their children. Natalie Bridger, a 35-year-old teaching assistant from Newcastle, was horrified to find out that her 12-year-old son Christopher had been watching porn — and showing it to his nine-year-old sister.

'Four or five weeks ago, as the family were all watching TV, my husband Lee and I noticed that our daughter Abigail had made gestures near her mouth which were clearly mimicking oral sex,' says Natalie.

'We both stopped dead and asked her what she was doing. She said, 'Oh nothing', but I caught her exchange a knowing glance with our son.

'After pressing him, he admitted that he had been looking at porn and Abigail had walked in and he'd shown her the images.

'Lee and I went cold. Neither of us knew how to block websites until recently and even now we do, we still need to know which ones to block.

'Every time he's on his tablet now, we wonder what he's doing. We've told him that we will check his history and we can take his tablet from him at any time to check what he's doing. So far, that seems to have worked.

'But the fact he's looking at it terrifies me. You can't stop children from using the internet altogether, yet at the click of a button they can open up a world of disturbing images which I really don't want them to see.

'It seems that no matter what we do to protect them, they always find a way around it.'

Natalie can only hope that the images haven't harmed them. For it's only a matter of time before we find out just how much damage internet porn is doing to this young generation.