Former San Francisco 49ers quarterback Colin Kaepernick is suing the NFL after he says teams refused to hire him in retaliation for taking a knee in the national anthem - and experts say he could get as much as $30 million.

His lawyer Mark Geragos said in a statement: '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 theater imitating weeks ago - should not be punished.'

The statement also warned that Donald Trump's recent calls for the NFL to fire players who protest peacefully 'threatens all patriotic Americans and harkens back to our darkest days as a nation.'

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Colin Kaepatrick (left and right), who ended his 49ers contract in March, has not been picked up by other teams; he says he's been blackballed for kneeling through the national anthem

The player filed a grievance with the NFL. His lawyer's statement blasted Donald Trump, who demanded Kaepernick and other protesting players be fired, for 'partisan political provocation'

Geragos said in the statement, which was posted on Twitter on Sunday, that the grievance was filed 'only after pursuing every possible avenue with all NFL teams and their executives.'

HOW MUCH COULD KAEPERNICK WIN? It's unclear how much Kaepernick expects to get if he wins; the figure, based on the money lost due to the collusion, would be estimated by the case's arbitrator. That figure would then be doubled under collusion rules, or trebled if the defendants are repeat offenders - though it seems unlikely that any are. The arbitrator could look to the salaries of free agents who were signed in 2017 as 'comparables'. Mike Glennon, 27, signed a three-year deal with the Chicago Bears for $45 million; Brian Hoyer, 31, signed a two-year deal with the 49ers for $12 million; Jay Cutler, 34, signed a one-year deal with the Miami Dolphins for $10 million; and Josh McCown, 37, signed a one-year deal with the New York Jets for $6 million. Kaepernick, who scored 16 touchdowns against four interceptions last year, is arguably better than them. Source: Sports International Advertisement

Kaepernick, 29, 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 black Americans by police.

Those protests then spread to other teams, causing consternation among some fans, who have interpreted the protests as being disrespectful to the flag and America itself.

Other players have continued the protests this season, prompting an angry response from Trump, who said players should be fired for not standing during the anthem.

Kaepernick's statement said that 'athletes should not be denied employment based on partisan political provocation by the executive branch of our government.'

Kaepernick opted out of his contract with the 49ers at the end of last season and remains a free agent despite having his best statistical season last year.

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.

Eric Reid (seen kneeling, far left) said he also thinks Kaepernick has been blackballed. Kaepernick began the kneeling protests as a response to police violence on black people

'I think all the stats prove that he's an NFL-worthy quarterback,' Reid continued, 'so that's his choice and I support his decision. We'll just have to see what comes of it.'

WHAT KAEPERNICK NEEDS TO PROVE If he hopes to win, Kaepernick will have to prove that more than one team - or a team and the NFL office - actively worked together to exclude him. If a single team's officials excluded him on the basis of his political actions, that would not count as collusion. He must also produce evidence of the collusion from within the last 90 days, as that is the limit set by the NFL's collective bargaining agreement. But that evidence must prove that the teams agreed to exclude him; merely criticizing his actions isn't enough; neither is a team hiring a statistically inferior quarterback over him. And despite being mentioned as a factor in the statement, Donald Trump is irrelevant with regard to a suit; collusion must occur within the NFL. Even if Trump himself persuaded a team not to hire Kaepernick in person, it wouldn't count. Making it harder for the player, rules for arbitration committees mean he can't subpoena emails or other communications, because such federal rules do not apply. Despite that, he still needs to prove to be arbitrator that the 'clear preponderance of the evidence' shows collusion to have taken place. Even if he is hired by another team, Kaepernick can continue his grievance - there is no requirement for him to drop it. Source: Sports Illustrated Advertisement

The NFL players' union said it would support the grievance, which was filed through the arbitration system that's part of the league's collective bargaining agreement.

'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,' Geragos said.

Geragos, a well-known celebrity lawyer, is best known for his work with high-profile clients such as Michael Jackson, Chris Brown and Winona Ryder.

The NFL Player's association said that it was not notified ahead of time about the filing.

'We first learned through media reports today that Mr Kaepernick filed a grievance claiming collusion through our arbitration system and is represented by his own counsel,' it said in a statement.

'We learned that the NFL was informed of his intention to file this grievance before today.'

According to Sports Illustrated, collusion occurs in the NFL 'when two or more teams, or the league and at least one team, join to deprive a player of a contractually earned right.

'Such a right is normally found in the collective bargaining agreement [CBA] signed by a league and its players' association.

'For example, the right of a free-agent player to negotiate a contract with a team cannot be impaired by a conspiracy of teams to deny that a player a chance to sign.'

NFL commissioner Roger Goodell has denied that Kaepernick was the subject of any kind of team collusion. It's been alleged that the NFL folded under pressure from Donald Trump

NFL commissioner Roger Goodell told ESPN last year that Kaepernick was not the subject of any kind of team collusion.

Goodell said that all teams will 'do whatever it takes to make their football team better. So those are football decisions. They're made all the time.

'I believe that if a football team feels that Colin Kaepernick, or any other player, is going to improve that team, they're going to do it.'

However, the movement started by Kaepernick has continued to prove controversial, and has lead to many fans threatening to boycott teams and players.

On Tuesday, Goodell sent a memo to players warning them that 'that everyone should stand for the National Anthem,' and adding that the league needs to 'move past' the controversy surrounding player protests.

Trump has made much play out of the controversy, which has energized his fanbase.

In March he told a rally in Kentucky that NFL owners 'don't wanna pick [Kaepernick] up because they don't wanna get a nasty tweet from Donald Trump.'

'Do you believe that?' he asked the cheering crowd. 'I said I'm gonna report that to the people of Kentucky, because they actually like it when people stand for the American flag.'

And on Wednesday he told Fox News' Sean Hannity: 'The NFL should have suspended [Kaepernick] for one game and he would never have done it again.

'They could have then suspended him for two games and they could have suspended him again if he did it a third time, for the season, and you would never have had a problem.'

Last week president Donald Trump said that the NFL should have suspended Colin Kaepernick the first time he kneeled for the National Anthem

Early Sunday morning Kaepernick retweeted three messages in support of the Black Panther party, in recognition of the 51st anniversary of its founding; this was one of them

Kaepernick himself did not comment on the grievance, but has continued his political activism on Twitter.

Early Monday morning he retweeted a series of remarks made by a black academic regarding the Black Panther party.

'51 years ago to this day, the Black Panther Party was born out of necessity,' wrote @LeftSentThis in one of the Sunday tweets. '51 years later it's still a necessity to fight police terrorism.'

The other tweets were the 10-point plan drafted by the Black Panthers' founders, and one of the organization's rules, which calls for members to remain politically aware.