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Parents have always worried about their children accessing inappropriate material and images on the web, but the rise of smartphones and tablets means parents have even less control over their offspring's online activities.

When it comes to teenagers' attitudes to online pornography, is sex education at school the key to making it a turn-off?

found that children are spending more time online, are more likely to go online alone and are using a wider range of devices to access the internet.

In 2012, 28% of children aged five to 15 owned a smartphone and among those aged 12 to 15, this rose to 62% - an increase of 21% on the previous year.

Fifteen per cent of girls aged 12-15 say their phone is the device they use most often to go online at home.

If teenagers are surfing the web alone in their bedrooms then there is a risk they could be exposed to online pornography - this is the generation for whom social networking sites, webcams and photo-sharing are all a normal part of growing up.

According to a report from Middlesex University this year, commissioned by the Children's Commissioner for England, "a significant number of children access pornography".

For Dr Miranda Horvath, senior lecturer in psychology and co-author of the report, porn is defined as "full explicit shots of sexual acts and genitalia" - not just a picture of a naked woman on page three of the Sun, although many studies of this subject do not give definitions.

The report found that pornography influences attitudes towards sex and relationships and could lead to young people having sex at an earlier age.

Yet experts say there is very little evidence for how it impacts directly on behaviour, nor which groups are most likely to access it.

Dr Horvath says the key is to provide children and young people with the space to ask questions about pornography and talk about their experiences of it.

"Young people have so much to say on this issue. They should be at the centre of the research - but much research is not designed with children in mind."

Her research shows that boys and young men are more likely to seek out porn than girls and young women. Females are more likely to have unwanted exposure to it.

The motivations for accessing it include curiosity, pleasure, peer influence and as a source of information.

Dr Horvath believes that mandatory sex education lessons at school are an essential starting point for getting children to talk about sex and relationships, which could reduce the appetite for sexually explicit material.

"The lessons should be non-judgemental and age-appropriate and not just a one-shot thing."

Dr Mark Limmer, a lecturer in public health at Lancaster University, agrees that sex and relationship education in schools should give out positive messages about sex.

"We do need children and young people to understand that sex has its place in a relationship, that it's pleasurable and intimate. Schools should take a healthy perspective on it.

"Somehow we're always saying to children 'don't do something'... We should help and support them rather than saying 'don't do that!'."

He suggests that boys and girls can become drawn to pornography if their questions about sex and relationships are not answered in the classroom in the same way that strong messages on smoking can encourage, rather than discourage, teenagers to try the habit.

He says it starts with lessons on gender equality in pre-school and talking differently about sex.

"If we get those bits right then it makes later conversations easier," Dr Limmer says.

Lucy Emmerson, from the Sex Education Forum, supports good quality sex education in schools which deals with a whole range of topics including consent, body image, gender and power imbalances.

But, she says, a third of schools currently have inadequate sex and relationship education according to Ofsted.

"Learning about these issues will equip young people to be more in control and the chances are they will be less likely to look for answers about sex and relationships elsewhere."

In her view, it all begins with teaching primary school children the proper names for their body parts, so that adults can feel comfortable discussing them too.

Then children should be taught about what is legal and what is illegal and the dangers of 'sexting' - sending sexually explicit messages and photographs between mobile phones.

"It's very important to be proactive. We can't wait for them to find pornography. Instead, let's talk about it openly."