A $760m settlement between the National Football League and thousands of former players, who contend the league downplayed the risk of concussions, was rejected by a US judge who said it might not be enough to pay all of the affected players.
The proposed deal, reached in August, had set aside up to $5m for each former player diagnosed with a certain brain condition as a result of their years on the playing field.
More than 4,500 former players were named plaintiffs in the lawsuit and up to 20,000 could ultimately be eligible for payment.
"I'm primarily concerned that not all retired NFL football players who ultimately receive a qualifying diagnosis, or their related claimants, will be paid," US District Judge Anita Brody wrote in papers filed in federal court in Philadelphia.
"Even if only 10 per cent of retired NFL players eventually receive a qualifying diagnosis, it is difficult to see how the monetary award fund would have the funds available over its lifespan to pay all claimants at these significant award levels."
Brody called on the NFL and plaintiffs to submit documentation that they believed showed the money set aside was adequate to meet the potential need.
But outside legal observers said the league may need to raise its settlement offer to win the judge's approval.
"They are going to have to come back with a different settlement," said Joseph Farelli, a partner in the New York law firm Pitta & Giblin, which specialises in labour law.
"She's saying the amount is not going to cover the people they say are going to be covered by the settlement."
The lawsuit, filed in 2012, contended that the league hid the dangers of brain injury among players while profiting from the sport's sometimes violent physical contact.
When the settlement was first disclosed in August, sports business experts described it as a modest amount of money for the NFL, which is believed to generate total annual revenue of $9bn or $10bn.