It's a typical Friday-night scenario, played out in homes across the country: a teenager is hurling abuse at her parents over their refusal to allow her boyfriend to spend the night in her bedroom. The scene often ends with the daughter flouncing off into the night, under-dressed and uncaring, into his arms. “You might as well let me. I'll do it anyway. And at least you'll know where I am” is the kind of refrain that strikes dread into the hearts of mums and dads everywhere.

As if the primal scream of teenagers rebelling against their midnight curfew or their onerous homework were not enough, parents find that, just as their children start out on a toxic hormonal journey, they have to grapple with the issue of sex.

So when is a teenager old enough to have a sleepover with the opposite sex - or even to entertain them in their bedroom with the door closed? And, given that it is illegal to have sex before the age of 16, how, as a parent, do you square the circle of wanting your child to be safe without collaborating in an illegal act?

For John, 45, a father of two adolescents and stepfather of another two, it is all about communication. “We make sure they know what's expected,” he says. “So we tell them that we're worried about them getting pregnant or STIs and that sex is part of a loving relationship. But at the same time, we have to acknowledge that it's natural for them to want to explore.” John's 17-year-old stepson was allowed to have his girlfriend sleep over - with her parents' permission - but for his 16-year-old daughter the issue has been more of building her self-confidence, so that she is under no pressure to have sex before she is ready.

But, in spite of the risks, many British parents appear content to adopt a more relaxed attitude or to ignore their children's relationships altogether rather than confront them. In a recent poll of 1,770 girls aged between 12 and 18 for the teen magazine Sugar, one in ten was allowed to have boys stay overnight in their bedroom, a third could take boys into their rooms and close the door during the day and 16 per cent had boys to stay overnight in separate rooms. Of those questioned, 262 girls admitted to having sex. Of them, nearly two-thirds had been under age.

Most parents pray that 13-year-old Alfie Patten and Chantelle Steadman, 15, who has just given birth to their child, will remain the exception, but the latest figures published by the Office for National Statistics make uncomfortable reading. The number of teenage pregnancies in England and Wales is rising, for the first time in five years. The data published last week shows that there were 41.9 conceptions per 1,000 15 to 17-year-olds in 2007 and nearly 8,200 pregnancies in girls between 13 and 15 - the highest figure in a decade.

Honor Rhodes, director of development at the Family and Parenting Institute, says that talking about sex is the key to your children's protection. Yet in spite of all the sexualised imagery bombarding them daily, we still feel uneasy about approaching the subject. “We feel able to say that you shouldn't dance in the middle of the motorway, but when it comes to talking about sex we get ridiculously squeamish,” she says. “People insist that the world is a very different place but teenagers still need boundaries.”

To help the conversation along, ministers plan to introduce compulsory sex and relationships lessons for children as young as 5 from next year. Last week the Government also announced £530,000 of funding for Speakeasy, a new free course by the FPA (formerly the Family Planning Association) giving parents and carers the information and skills necessary to talk to their children about sex and relationships.

There is widespread agreement that sex must be discussed within the context of committed loving relationships, and that the best place to start is at home. As Honor Rhodes advises, you should “give children timely information - talking about sex like you do about mountains, in a boring factual way, from a very young age”.

For those parents who think that they can monitor their child's sexual relationships when they reach puberty, the case of one Scottish mother is a cautionary tale. Last month Daniel Balfour, a 22-year-old man from Berwick-upon-Tweed, was given three years' probation and community service for having sex with a 14-year-old girl. The two had had sex in the home of the girl's parents, sanctioned by her mother.

“Do you know what your 14-year-old is doing,” wrote the girl's mother in her local paper, “well I do. As many mothers know, you can't lock a 14-year-old in their room until they're 16. I would rather know where my daughter is and who she is with than have her running round the streets getting into trouble.”

Some may sympathise with this sentiment, but Professor Tanya Byron, psychologist and times2 columnist, says that no 14-year-old she has met is emotionally or psychologically mature enough to understand the implications of sex in the longer term. “In this case, it may have been the teenager's way of crying for someone to say 'Stop!',” she says. “In the way that toddlers up the ante and get more out of control until boundaries are set, teenagers also need boundaries to feel settled.”

If you leave it until the moment when your teenager asks to sleep over with their boyfriend or girlfriend then it is probably too late to supervise their behaviour. As teenagers approach 16, and the age of consent, they are becoming young adults and are inclined to reject the family unit and explore. Professor Byron urges parents to find a balance between over-protecting their child and abolishing all boundaries.

“If a mother has had a bad early sexual experience, she may wish that her parents had given her more boundaries and managed her better, but if she runs in with all guns blazing and foists her anxiety on to her child, she'll lose the battle before she starts,” she says, adding that it can help sometimes to share experiences and explain why you are so concerned.

While parents wishing to control their teenager's sleepovers may like to take in cups of tea as a way of checking that nothing untoward is going on in a child's bedroom, for many parents the law may be a threat but it can also be a comfort. Honor Rhodes insists that in the end parents must become tougher and face up to their responsibilities.

“We really have to be prepared to be disliked,” she says. “We are reluctant to say no and I really think we should. Children have lots of friends, and what they need from us are parents.”

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