The NFL Players Association filed a grievance Tuesday against the league's new national anthem policy, approved by club owners in May in response to criticism from US President Donald Trump.
The policy adopted by the league requires players and all team personnel on the sidelines to stand during the traditional playing of the national anthem, "The Star-Spangled Banner."
Players have the option of staying in the locker room while the anthem is played under the policy, which was approved without input from the players because pre-game operations do not fall under the collective bargaining agreement between the union and the NFL.
Players who go onto the field and don't stand face a punishment, although New York Jets chairman Christopher Johnson said he would pay any fine incurred by one of his players.
"The union's claim is that this new policy, imposed by the NFL's governing body without consultation with the NFLPA, is inconsistent with the collective bargaining agreement and infringes on player rights," the union said in a statement.
"In advance of our filing today, we proposed to the NFL to begin confidential discussions with the NFLPA executive committee to find a solution to this issue instead of immediately proceeding with litigation. The NFL has agreed to proceed with those discussions and we look forward to starting them soon."
The talks will begin later this month.
The move comes one day before the Baltimore Ravens become the first NFL team to open pre-season training camp as rookies report Wednesday to begin workouts ahead of pre-season games in August and the September start of the 2018 campaign.
Former San Francisco 49ers quarterback Colin Kaepernick began the anthem protest in the 2016 pre-season objecting to social injustice and racial inequality ossues in the wake of police shootings of unarmed African-American men.
Other players adopted the move after Kaepernick went unsigned for the 2017 campaign, but major attention was drawn to the move when Trump derided protesting players and said they should be fired, calling any protester kneeling for the anthem a "son of a bitch."
Trump portrayed the protesting players as disrespectful to the flag and nation and cheered the NFL's anthem policy, saying "NFL owners did the right thing" while adding, "You have to stand proudly for the national anthem or you shouldn't be playing, you shouldn't be there. Maybe you shouldn't be in this country."
The NFLPA argues that kneeling during the national anthem is not "conduct detrimental" to the league, which would provide NFL commissioner Roger Goodell the power to impose the policy, and punishing peaceful demonstrations could be used to ban such actions as prayer.
Under the National Football League's new policy, players who go onto the field and don't stand for the national anthem face a punishment