Under-18 rugby players should be barred from the scrum front row because they are at a higher risk of suffering neck injuries, a study suggests.
Research has shown younger players often do not have the same neck strength as seniors.
A team at Edinburgh University is advising that youth players undergo tests to demonstrate they have the necessary neck strength before being approved to play adult rugby.
They tested the physical strength of adult players from amateur leagues and under-18 front row players and found that despite looking as strong as their senior counterparts, the lesser strength and fatigue endurance of youths put them at a "significant disadvantage".
Dr David Hamilton, a researcher in the department of trauma and orthopaedics, said: "In the test group of high-performance under-18 group players, only two out of 30 players recorded the average neck strength of the adult group."
The findings have been welcomed by Scottish Rugby, which assisted with the study aimed at helping reduce injuries among school-age players.
Scrums are responsible for a significant proportion of spinal injuries, but the national rugby body has already adopted new guidelines that have seen the serious injury rate fall.
The changes followed several serious injuries suffered by young players. In 2009, a 14-year-old at Stewart's Melville lost the use of an arm after suffering a neck injury, while in 2008, a 17-year-old at Merchiston Castle suffered serious spinal injuries.
Hamish Simpson, professor of orthopaedic surgery, said: "Our results showed that although under-18 players were as strong as the adults in general, they were unable to generate the same neck muscle force as adult players.
"It is likely that weak necks are a risk factor for the scrum collapsing - an event associated with serious neck injury risk. To ensure the safety of all six front-row players, it is essential they are all strong enough to compete safely."
Dr James Robson, Scottish Rugby's chief medical officer and doctor on the past six British & Irish Lions tours, said: "This study suggests youngsters can achieve peripheral strength. However, the key area for us is the strength of their neck and it would appear it's very difficult to attain 'adult-type strength' in this particular area.
"This research is helping to underpin our safety policies. It validates our stance on where and when we allow under-18s to play in senior rugby."
Last year a school that has developed some of Scottish rugby's most famous names withdrew from several fixtures amid fears the difference in size between players could lead to its pupils being injured.
Glenalmond College, Perthshire, which counts former national captains David Sole, David Leslie, and Rob Wainwright among its alumni, took the decision based on the risk faced by small and light members of its first team.
The study was funded by Edinburgh University and appears in the British Medical Journal.