The National Football League on Wednesday announced that it would impose its most stringent rules to date on managing concussions, requiring players who exhibit any significant sign of concussion to be removed from a game or practice and be barred from returning the same day.

The league has maintained, despite widespread criticism, that it was safe to allow players to return to the field as soon as their symptoms were gone  even in the same game in which the injury occurred. It has not been uncommon for players to return, either after passing a sideline test or by not revealing their symptoms.

The announcement is the latest of several recent changes to how the N.F.L. handles concussions, and comes at a time when some of its most high-profile players are coping with head injuries, including the star quarterbacks Ben Roethlisberger of the Pittsburgh Steelers and Kurt Warner of the Arizona Cardinals.

The new rule was made in the wake of heightened scrutiny of the league’s approach to head injuries, outlined in a series of articles in The New York Times. At a Congressional hearing in October, the N.F.L. was criticized for its practices and research methods. Last week, the co-chairmen of the league’s committee on concussions resigned under fire, on the same day the league announced that players who sustain concussions could be cleared to return only by independent experts.