A red card given to England full-back Freddie Steward in Saturday's defeat by Grand Slam winners Ireland has been called "an utter farce" and "absolutely ridiculous" by former players.
Freddie Steward was sent off just before half-time after his elbow made contact with Hugo Keenan's head.
Keenan left the field for a head injury assessment, which he failed.
Former England scrum-half Matt Dawson said: "The officials are showing a lack of understanding of the game."
Dawson argued that Steward was trying his best not to make contact as he and Keenan flew towards each other.
"I can't disagree with how the contact is a red but it is the context of how it went to that contact," Dawson said on BBC Radio 5 Live.
"He is stepping and slowing and turning to get out of the contact area, and has been punished by being sent off. It is a mockery.
"He was doing his utmost to avoid any kind of contact. It is an utter farce."
Stricter refereeing around dangerous challenges is part of World Rugby's efforts to reduce head injuries, but critics have raised concerns over red cards deciding the outcome of matches. Ireland led 10-6 when Steward left the field and went on to win 29-16 in Dublin.
England captain Owen Farrell said the red card for Steward "seemed harsh", but added: "It's not for me to have an opinion on. They made the rules for a reason, that's all I can say on it."
World Rugby, England's Rugby Football Union and the Welsh Rugby Union are facing legal action from more than 200 ex-professional players who say they were not sufficiently protected from brain injuries in the sport.
As well as tightening tackle laws in the elite game, World Rugby recently began a trial to reduce tackle height in the community game. A 20-minute red card has been trialled in the Southern Hemisphere's Rugby Championship.
Steward's was the third red card of the 2023 Six Nations, with France and Scotland receiving one each in their third-round match.
Twelve yellow cards were shown during the tournament - including one for England's Jack Willis in the final five minutes against Ireland.
'Ridiculous' and 'a shambles' - how rugby reacted
World Cup winner Jason Robinson described referee Jaco Peyper's decision to give Steward a red as "absolutely ridiculous", adding: "That was a yellow card at best."
England centre Jonathan Joseph said it was an "absolute shambles", while former wing Ugo Monye described it as a "bad call" because the clash was "a rugby incident".
Ex-England captain Will Carling said: "Steward had a split second to react - after a knock-on - to a man running at him.
"There was no intent - he was protecting himself - what a joke."
England's Test cricket captain Ben Stokes also called the decision a "shambles", adding: "Rugby [is] being ruined."
Former Ireland back row Jamie Heaslip was sympathetic to England's plight, but believed World Rugby's head contact process, which looks at the degree of danger in the collision and whether there was any mitigation, was followed correctly.
"In my opinion it is a rugby incident, it happens but once he [the referee] goes to the protocol, it is not going to go good."
Player welfare group Progressive Rugby, which has called for more action to protect players from injury, said it was "a tough call", adding that it was "good to see" Keenan receive a head injury assessment.