Large bottles of cheap cider were associated with drinking in public
Parents could help keep their teenagers away from drink-fuelled violence and sex by giving them a weekly alcohol allowance, a study has suggested.
A third of those polled had experienced violence when drunk and 12.5% reported sexual encounters they regretted.
University researchers questioned nearly 10,000 15 to 16-year-olds in the north-west of England.
They got into trouble more when buying their own cheap alcohol, rather than getting access from parents, it found.
Carefully introducing alcohol to children may help them "prepare themselves for life in an adult environment dominated by this drug", said the study.
Study leader Professor Mark Bellis, from Liverpool John Moores University, said the negative impacts of alcohol on children's health were "substantial".
"Those parents who choose to allow children aged 15 to 16 years to drink may limit harm by restricting consumption to lower frequencies - for example, to no more than once a week - and under no circumstances permitting binge drinking.
"However, parental efforts should be matched by genuine legislative and enforcement activity to reduce independent access to alcohol by children and to increase the price of cheap alcohol products."
In similar studies done by the university in 2006 and 2007 researchers concluded that teenagers who drank alcohol with their parents in moderation were less likely to binge drink.
In 2008 their results showed that teenagers were drinking an average of 44 bottles of wine or 177 pints of beer a year each.
This year's survey found that teenagers who relied on obtaining their own supplies of cheap alcohol were much more likely to be involved in violence and other forms of bad behaviour.
As well as those reporting violence and sexual encounters they regretted, some 35.8% of the teenagers had drunk in public places like parks and shopping centres and 45.3% had suffered forgetfulness after drinking.
Researchers said that while no teenage drinking was risk-free, the way teenagers got hold of alcohol made a big difference to the harm caused.
Just under 20% of teens who drank once a week and were supplied with alcohol by their parents had been involved in violence when drunk.
The proportion getting into fights rose to 36% for those drinking as often, after obtaining alcohol by other means.
A strong link was found between the availability of cheap drinks and alcohol-related violence, "regretted" sex, and drinking in public places.
Large bottles of cheap cider were particularly associated with drinking in public, while relatively expensive alcopop-style drinks were less of a significant problem, said researchers.
Writing in the journal BMC Public Health, they warned that parents who tried to impose alcohol bans might only shift the problem away from the family into the street.
They added: "Our results suggest that such a move, even if overall consumption did not increase, could exacerbate negative outcomes from alcohol consumption among teenagers."
"Despite much of the chronic burden of alcohol-related disease falling on adults, the foundations of such damage are often established in childhood," they added.
In their paper the scientists said alcohol had emerged in recent decades as "one of the major international threats to public health".