The World Rugby Council announced the amendment to Law 8 following a recommendation by the international federation's rugby committee and laws review group.
With post protectors having increased in size for safety reasons and defenders legally obliged to stay behind their own line, it was agreed that it had become increasingly difficult for players to legally defend the area and the posts.
The rule change has come into effect immediately as players worldwide wait to learn when they may be able to return to action amid the coronavirus crisis.
World Rugby chairman Bill Beaumont said: "World Rugby's mission is to make the game as simple, safe and enjoyable to play as possible. This law amendment reflects that mission.
"By stipulating that an attacking team can no longer score against the post protector and therefore must ground the ball in-goal, this gives defending teams a fair chance of preventing a try from being scored."