Deputy Prime Minister Nick Clegg will today launch a sex abuse awareness campaign

Nick Clegg told a group of teenage girls he found it "shocking" that a third of their peers have reported they have experienced some form of sexual violence from a boyfriend on Monday.

Many young people dismiss the idea of "date-rape", believing the offence only occurs on rare occasions, and think that sexual attacks are more likely to occur at the hands of a stranger, according to the government.

Visiting a rape crisis centre in Ealing, west London, the deputy prime minister spoke to a group of 16, 17 and 18-year-old girls from a local school about the issues after they were shown a new government-funded TV advert which aims to challenge the myths around sexual attacks.

He told the schoolgirls: "I think, for far too many people, they think rape is something that happens when you meet a stranger in a dark alley who inflicts physical violence on you.

"What we're trying to do with this advert is make sure that people know it's about, maybe, familiar relationships with people you've known for a long time, and maybe there's no violence involved at all.

"That's why it's so hard-hitting, because we really need to shift the way people think about this."

A cinema and online advertising campaign are also part of the coalition push to address the issue. Funded by the Home Office, it builds on the government's wider This Is Abuse campaign, which aims to help teenagers develop healthy relationships.

Clegg told reporters: "We've got such a highly sexualised culture at the moment and there are huge pressures.

"Any parent of a teenage girl will tell you there's huge pressures on teenage girls to conform to behave in a particular way, and I just think it's really important we give them the protection they need that when they say 'no' it means 'no' and the law is on their side.

"It's not normal, frankly, for governments to do things like this or for politicians to talk about it. But I think the scale of the problem now is big, and it's a really big problem for a lot of teenagers and we need to address it."

He added: "What this ad campaign is seeking to achieve is to send an incredibly strong message to teenagers, and particularly to teenage boys, that, if you have sex with a girl who doesn't want to and doesn't consent, you're committing rape - it's a crime.

"When a girl says 'no', no means no, means no. There's no ifs and buts.

"When you've got a situation where a third of teenage girls say they have been subject to sexual coercion and abuse, when lots of teenagers say in surveys that they think it's OK for a boy to expect to have sex with a girl they spend time with, something's going wrong and we need to challenge it, and this is a very hard-hitting advertisement which challenges all of that."

NSPCC research shows that the problem is not just confined to girls, with 16% of boys also reporting that they have experienced some form of sexual violence from a boyfriend or girlfriend.

It found that the highest proportion of sexual abuse - 66% - is perpetrated by young people under the age of 18.

The adverts are aimed at 13 to 18-year-olds and feature a teenage girl being coerced into sex by a boy as a party goes on downstairs.

The girl says "I don't want to", but the boy persists. A double of the boy then appears, watching the scene from behind an invisible wall.

The viewer is asked "If you could see yourself, would you see rape?"

The adverts will run for a month on youth TV channels, starting tonight, during programmes such as E4's Skins.

Equalities Minister Lynne Featherstone, who also took part in the discussion with the youngsters, said: "Teenagers are inundated with information about relationships, from their friends, the internet and TV.

"This campaign aims to dispel the myths that can lead to acceptance of rape in relationships.

"Bringing the issue out in the open will help teenagers feel confident about challenging abuse when they see it and ultimately protect potential victims."

Darya Keivani, 16, said: "The fact it was from the boy's perspective was good. Often things like this focus on women being more aware of their surroundings, it's good that it shows it happening with a woman with someone she thinks she can trust."

Yasmeen Razaq, 18, added: "I thought it was very effective. It shows that even if it's your own boyfriend, he may pressure you to have sex. It's a rape even though it's your own boyfriend."

But Holly Dustin, director of the End Violence Against Women Coalition, criticised the fact that the campaign was not being run in schools directly.

She said: "Young people are bombarded with messages that blur the lines of what sexual consent means, so we fully support this Home Office campaign and welcome the fact that the Home Office has worked closely with experts in the women's sector in developing it.

"Unfortunately sexual violence and abuse is common in young people's relationships and has an impact on physical and emotional health, on educational attainment and is a considerable drain on the public purse.

"It is critical, therefore, that we invest in long-term campaigns like this to change attitudes and behaviours, similar to drink-driving campaigns, and that we ensure funding for women's services who provide support."

Questioned about this, Clegg said: "Teenagers rely so heavily on what they see in the media, what they see on the internet, what they find on their mobile phones, what they say to each other when they're out and about when they're in the playground and so on - this is directed at them, it's a very, very direct advertising campaign."

Leonie Hodge of Family Lives said 70% of all rapes are carried out by someone known by the victim and said many of the young males she has spoken to do not understand what rape is.

She said: "This advert is really strong because it actually dispels the myths about rape and it says to people 'you could be a victim of this', it could happen to anyone, and also for young males, 'you could be a perpetrator of this', if you're putting too much pressure on and you're forcibly getting what you want, that is rape, there's no consent."

Jon Brown, head of the NSPCC's sexual abuse programme, said: "From calls to our ChildLine we know that many young people misguidedly accept this as part of a relationship and so do nothing about it.

"We have to change that view, through education and advice, because this is sexual abuse and should not be tolerated under any circumstances.

"Our research has shown that at any one time there may be as many as 250,000 teenage girls suffering in this way but are too embarrassed or frightened to say anything about it.

"They will report a sex assault if it's committed by an adult but when another teenager carries out the offence the same alarm bells don't ring and they suffer in silence."