Former San Francisco 49ers quarterback Colin Kaepernick has sued the the owners of the National Football League for colluding against him.

Kaepernick, 29, has been without a team since he opted out of his contract with the 49ers in March.

Kaepernick started a national conversation about political activism by athletes last season when he decided to sit, and then kneel, during the anthem to bring attention to mistreatment of African Americans by police.

He first protested by sitting during the national anthem in August 2016, before opting to kneel instead.

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Other players followed suit, and criticism from President Donald Trump this September saw the protests spread.

In a statement released on Sunday, Kaepernick’s lawyers wrote: “We can confirm that this morning we filed a grievance under the CBA on behalf of Colin Kaepernick.

“This was done only after pursuing every possible avenue with all NFL teams and their executives.

“If the NFL (as well as all professional sports leagues) is to remain a meritocracy, then principled and peaceful political protest – which the owners themselves made great theatre imitating weeks ago – should not be punished and athletes should not be denied employment.

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“Such a precedent threatens all patriotic Americans and harkens back to our darkest days as a nation. Protecting all athletes from such collusive conduct is what compelled Mr Kaepernick to file his grievance.

“Colin Kaepernick’s goal has always been, and remains, to simply be treated fairly by the league he performed at the highest level for and to return to the football playing field.”

Speaking to BBC Sport last week, Robert Kraft, owner of the New England Patriots said he had “never heard anyone talking about excluding” Kaepernick.

San Francisco safety Eric Reid, Kaepernick’s former teammate, has been kneeling during the anthem before games, including Sunday’s 26-24 loss at the Washington Redskins.

“I’ll have to follow up with him,” Reid said after the game. “It sure does seem like he’s being blackballed. I think all the stats prove that he’s an NFL-worthy quarterback. So that’s his choice and I support his decision. We’ll just have to see what comes of it.”