World Rugby chief executive Brett Gosper has urged referees to show more red and yellow cards in a continued effort to put an end to dangerous tackles in the game.
The governing body has been on a campaign to eradicate dangerous tackles, particularly those that impact on the head, as the problem of concussion continues to blight the game.
"The cards are there to change behaviour," Gosper told the Daily Telegraph.
"They only continue to be a problem if behaviour does not change. The only way you can get player behaviour to change is to sanction with red cards and actually, we have probably not seen enough of it.
"I would say in many ways we have probably not been hard enough. We have not had the behaviour change that we are seeking yet, so we have to continue in that vein."
Gosper said not enough has been done to prevent injuries from collisions in the professional game and it was now up to the referees to make tougher calls on the field.
His comments come after several contentious tackles during the November internationals, including England Owen Farrell's tackle on South Africa's Andre Esterhuizen which went unpunished despite a video review.
Australia's Samu Kerevi also escaped punishment when he clattered into Leigh Halfpenny in Cardiff and left the Wales full back with concussion symptoms.
"The whole tackle law is to protect the ball carrier and the tackler, in fact mostly the tackler, given that two thirds of concussions occur to that player rather than the one carrying the ball," Gosper added.
"Dropping the height of the tackle is due to the statistics showing us that if the player is bent at the waist as they tackle, they are four times less likely to suffer a concussion."
