The NFL have taken drastic action to prevent on-field protests during the national anthem by introducing a new rule which will see teams fined if players kneel on the field.
The rule change, which was enacted by a vote of team owners during the Spring League Meeting, states players must either stand on the sidelines or remain in the locker room.
It states that "a club will be fined by the League if its personnel are on the field and do not stand and show respect for the flag and the anthem", and that commissioner Roger Goodell "will impose appropriate discipline on league personnel who do not stand and show respect for the flag and the anthem".
In a statement, Goodell said: "The policy adopted today was approved in concert with the NFL's ongoing commitment to local communities and our country - one that is extraordinary in its scope, resources, and alignment with our players. We are dedicated to continuing our collaboration with players to advance the goals of justice and fairness in all corners of our society.
"The efforts by many of our players sparked awareness and action around issues of social justice that must be addressed. The platform that we have created together is certainly unique in professional sports and quite likely in American business. We are honoured to work with our players to drive progress.
"It was unfortunate that on-field protests created a false perception among many that thousands of NFL players were unpatriotic. This is not and was never the case."
However, the NFL Players' Association issued a statement via social media saying it had not been consulted on the rule change and suggesting it could move to challenge it.
"The NFL chose to not consult the union in the development of this new 'policy'," the statement said. "NFL players have shown their patriotism through their social activism, their community service, in support of our military and law enforcement and yes, through their protests to raise awareness about the issues they care about.
"The vote by NFL club CEOs today contradicts the statements made to our player leadership by commissioner Roger Goodell and the chairman of the NFL's management council John Mara about the principles, values and patriotism of our league.
"Our union will review the new 'policy' and challenge any aspect of it that is inconsistent with the collective bargaining agreement."
The 2017 season was marked by protests during the pre-game anthems, with several players taking a knee to raise awareness of social and racial inequality in the United States.
The protests had been started by quarterback Colin Kaepernick in 2016, but grew in scope as US president Donald Trump called for players to be fired by teams for taking part - with a number of owners siding with their players and joining the protests - at least temporarily.
When addressing with the issue last year, Goodell had said the NFL would not force its players to stand for the anthem but believed that they should.