The NFLPA filed a grievance against the NFL on Tuesday, challenging the league's new policy regarding players and the national anthem.
The league announced in late May that players and team personnel on the sidelines must stand when the national anthem is played before. Individuals will be allowed to remain in the locker rooms, but teams will be fined if when on the field they "do not stand and show respect for the flag and the anthem".
Shortly after that announcement, the players union said its members were not consulted about the changes and it would review the new policy and "challenge any aspect of it that is inconsistent with the collective bargaining agreement".
The NFLPA also said the move contradicts statements commissioner Roger Goodell and New York Giants owner John Mara made to union leadership "about the principles, values and patriotism of our league".
Formally announcing its challenge to the policy, the NFLPA said in a statement: "Our union filed its non-injury grievance on behalf of all players challenging the NFL's recently imposed anthem policy. 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.
"In advance of our filing, 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 issue appears on the cusp of dragging into a third consecutive year, having come to the surface in the 2016 preseason, when then-San Francisco 49ers quarterback Colin Kaepernick kneeled during the national anthem to protest social injustice and racial prejudice in the United States.
The topic has dogged the NFL ever since, in part because US president Donald Trump latched on to the issue and has continued to criticise the players.
As recently as last week, Trump described the NFL's new policy of allowing the players to remain in the locker room as "worse" than protesting.
"Isn't that worse than not standing?" Trump said at a rally in Montana. "That doesn't play. I actually think in many ways its worse."
Meanwhile, the NFL is seeking to have dismissed a grievance filed by Kaepernick, in which he claims he has been blacklisted by the league and its owners and teams, since initiating his protest. He remains a free agent, having not been signed for the 2017 season either.
|Juventus 3 Chievo 0: Allegri´s men cruise despite Ronaldo penalty save|
|BREAKING NEWS: Boateng completes shock Barcelona switch|
|United Arab Emirates 3 Kyrgyzstan 2 (after extra time): Khalil penalty sets up Socceroos showdown|
|West Indies add Drakes to new-look coaching team|
|Barcelona is a huge opportunity – Boateng confirms shock move is close|
|Donnarumma wishes Higuain luck ahead of expected Chelsea switch|
|Qatar v Iraq: Katanec anticipating ´huge challenge´ in last 16|
|Nuno hit with FA misconduct charge for pitch celebration|
|Furious Carreno Busta and Zverev bow out, Djokovic marches on|
|#NotMartinFromWakefield – Siewert shows viral sensation the door|
|Djokovic feeling the strain after late-night Medvedev battle|
|Henderson sidelined again ahead of Six Nations|
|Australia 0 Uzbekistan 0 (aet, 4-2 on penalties): Ryan heroics see holders hobble onwards|
|Halep exit leaves four-way fight to be crowned number one at Australian Open|
|Super Bowl-bound Rams set for London return, Chiefs to face Chargers in Mexico|
|Huddersfield appoint Siewert as Wagner´s replacement|
|Genoa 0 AC Milan 2: Borini and Suso step up with Higuain left out|
|McTominay signs new Manchester United contract|
|Pizzi to stand down after Saudi Arabia elimination|
|Boateng closing in on shock Barcelona switch - reports|
|Catalans to defend Challenge Cup after resolving RFL dispute|
|Djokovic wears down Medvedev to reach last eight|
|Mouratoglou: I knew ´loyal´ Serena would not fire me after US Open turmoil|
|Barcelona B defender Wague accuses Ejea fans of racist abuse|
|I felt like I had been hit by a train - Halep upbeat despite losing Serena battle|
|Arsenal confirm Mislintat departure|
|Icardi will renew, 100 per cent - Wanda Nara offers latest Inter assurance|
|Japan 1 Saudi Arabia 0: Four-time winners send Pizzi´s men out|
|A souvlaki maybe named after him, a growing fan base – Tsitsipas creating Melbourne buzz|
|Barca winger Dembele out for 15 days|
|Strikers blown away by Hurricanes in Adelaide|
|Ireland lose Beirne for England and Scotland clashes|
|´Innate fighter´ Serena says character key to beating Halep|
|Hudson-Odoi ´an outstanding talent´ - Bayern great Ballack hails Chelsea youngster|
|Scotland call up four as hand injury forces Watson out|
|Serena comes through major Halep battle to reach last eight|
|Better than Rafa? Camacho cameo leaves Klopp unconvinced by transfer options|
|England job ´a privileged position´, says Southgate|
|Emery has confidence in Arsenal outcast Ozil|
|Zverev destroys racquet in Aus Open meltdown|
|From Villarreal to Aus Open quarters – Bautista Agut breaks major duck|
|Brees: I plan on being here next year and making another run at it|
|´Unf******-believable bro!´ - Brady stunned after reaching third straight Super Bowl|
|New England find answers, penalty costs Kansas City – three takeaways from Patriots´ win over Chiefs|
|Donald Trump congratulates Brady and Super Bowl-bound Patriots|
|Rose helps Timberwolves down Suns|
|Brady leads Patriots into third straight Super Bowl|
|Osaka overcomes Sevastova to reach quarters|
|Chelsea remain behind Sarri – David Luiz|
|McVay explains Gurley´s limited role in Rams´ win over Saints|
|Southgate hails ´tremendous talent´ Rashford|
|Long earns maiden PGA Tour win at Desert Classic|
|Flanagan resigns as Sharks head coach|
|Rams´ Robey-Coleman admits he committed penalty on controversial no-call|
|NFL admits they blew the call – Saints coach Payton|
|Terrible no-call, Goff shines – three takeaways from Rams´ win over Saints|
|Lonzo Ball to miss 4-6 weeks with ankle sprain|
|Patterson added to Australia Test squad|
|Zuerlein´s overtime kick sinks Saints and sends Rams to Super Bowl|
|It was a clear goal – Valverde dismisses Barca VAR controversy|
|Rashford can emulate Man United greats Ronaldo and Rooney – Solskjaer|
|Karmichael Hunt swaps Reds for Waratahs|
|Barcelona forward Dembele set for tests on sprained ankle|
|Neymar insists Barcelona and Real Madrid links not ´concrete´|
|England trio Nowell, Slade and Cowan-Dickie sign fresh Exeter deals|
|Windies coach Pybus not distracted by detractors|
|Barcelona 3 Leganes 1: Messi and Suarez combine to spare Barca´s blushes|
|I´m sure Michael would be extremely proud - Vettel hails Mick Schumacher|
|Young plays down injury fears for England trio Launchbury, Shields and Hughes|
|Pochettino: Lucas Moura fit for Chelsea clash|