The researchers theorise that the difference occurs because a mother's brain generates oxytocin when she cuddles and plays with her children.

London - Getting a reluctant child to give an elderly relative a kiss often requires some gentle persuasion.

But parents who force their sons and daughters to give granny a peck on the cheek may be doing them harm, it was claimed on Tuesday.

For instead of helping a child learn about showing affection, it may blur the boundaries of what is acceptable when it comes to physical contact, according to Lucy Emmerson, co-ordinator of the Sex Education Forum.

She even claims that encouraging a youngster to blow a kiss, high-five or wave to a relative instead will help them avoid future sexual exploitation.

Children need to learn from the start about the importance of consent and that ‘their bodies are their own’, she says.

Her controversial comments, in an online sex education resource for teachers, were immediately attacked by family campaigners. They said there was no evidence that children who are persuaded to kiss close relatives are more at risk of being sexually exploited later and said her recommendation undermined parents.

Norman Wells, director of the Family Education Trust, said: ‘Even if the distinction is lost on the Sex Education Forum, children and young people are able to recognise that there is all the difference in the world between self-consciously– and perhaps on occasion reluctantly – kissing an uncle or aunt on the cheek on the one hand, and accepting unwanted sexual advances on the other.’

Miss Emmerson made her comments online in the Sex Education Forum’s termly e-magazine. Discussing the National Survey of Sexual Attitudes and Lifestyles, which found that one woman in five and one man in 20 in Britain had experienced attempted sex against their will, she said it raised questions over when children should be taught about consent.

Saying that it showed non-consensual sex is widespread, Miss Emmerson added: ‘I believe learning about consent starts from age zero. Much is learnt by young children from everyday experiences about whether or not their opinion is valued and if they have any control over physical contact with others.’

She cited a recent blog by author Kasey Edwards, Stop Asking My Daughter To Give You A Kiss, which acknowledges that parents automatically comply with social rituals such as a child being told to kiss a relative, friend or even a stranger.

‘Intervening may be awkward... but it is necessary if we are truly to teach children that their bodies are their own and that their instincts should be followed,’ Miss Emmerson wrote.

‘Suggesting alternatives to the child such as a high-five, a hug, blowing a kiss or a wave put the child in control. If we can’t manage to create a culture of consent for everyday physical contact, it will be surely be a tall order for sexual situations.’

A survey of 890 young people, mainly aged 16 or 17, by the Sex Education Forum, showed that almost one in three had not learnt about consent at school.

Dr Meg Barker, a psychology lecturer at The Open University who co-edits the journal Psychology and Sexuality also warns in the e-magazine: ‘At the moment coercive practices are quite commonplace in everyday lives unfortunately, from friends and family members kissing and cuddling kids when they don’t want that to happen, to friends and family trying to persuade each other to engage in social events that they don’t want to go to through guilt and manipulation.’

Margaret Morrissey, of family campaign group Parents Outloud, described the recommendation as ‘ridiculous’.

Referring to kissing relatives, she said: ‘It’s something we need to preserve, because it’s part of being a caring family.

‘Parents are very sensible and know exactly what’s appropriate for their children. The Sex Education Forum is trying to take any kind of human feeling and kindness out of the way we bring up children and that’s really sad.

‘If a child isn’t taught through the family connection how to show appropriate affection, then they’re going to have a very difficult time when they become teenagers and adults.’