The move is expected to transfer the labor impasse from the bargaining room to the courtroom. The owners previously said they would lock out the players if the sides failed to come to terms on a new collective bargaining agreement before Friday's midnight deadline.
Were a lockout to happen, the players would sue the league for restraint-of-trade antitrust violations. Star quarterbacks Peyton Manning, Tom Brady and Drew Brees already have filed as the lead plaintiffs, although many more players could be involved.
NFL lead negotiator Jeff Pash said no decision has been made whether to lock out the players.
MICHAEL McCANN: Road map to what happens next
The owners made a CBA proposal to the union earlier Friday, but the union did not accept it, presumably because the owners have refused to turn over team-by-team financial information the union has requested.
At roughly 4:45 p.m. ET, executive director DeMaurice Smith met with the media and said if the league wanted to extend the negotiating deadline for a third time in a week, it would have to turn over 10 years of audited team financials by 5 p.m. Smith did not threaten decertification, but the threat was implicit.
"We met with the owners until about 4 o'clock today," Smith said outside the mediator's office. "We discussed a proposal they had presented. At this time, significant differences continue to remain. We informed the owners that ... if there was going to be a request for an extension, that we asked for 10 years of audited financial information to accompany that extension."
About 15 minutes later, the union decertified.
NFL statement: Union left 'very good deal on the table'
Also in the NFL's offer, according to the league:
• Maintaining the 16 regular-season games and four preseason games for at least two years, with any changes negotiable.
• Instituting a rookie wage scale through which money saved would be paid to veterans and retired players.
• Creating new year-round health and safety rules.
• Establishing a fund for retired players, with $82 million contributed by the owners over the next two years.
• Financial disclosure of audited league and club profitability information that is not even shared with the NFL clubs. That was proposed by the NFL this week, and rejected by the union, which began insisting in May 2009 for a complete look at the books of all 32 clubs.
The last work stoppage came when the players struck 24 years ago, leading to games with replacement players.
In 1989, the NFLPA also decertified. Antitrust lawsuits by players forced a new CBA in 1993 that included free agency, and the union formed again that year.
"The parties have not achieved an overall agreement," federal mediator George Cohen said, "nor have they been able to resolve the strongly held competing positions that separated them on core issues.
"No useful purpose would be served by requesting the parties to continue the mediation process at this time."
The players' union immediately shut down its websites -- NFLPA.org and NFLPlayers.com. A search for NFLPA.org yielded this message: "Error 404: Football Not Found. Please be patient as we work on resolving this. We are sorry for the inconvenience."
GALLERY: RARE PHOTOS FROM THE 1987 SEASON
When Commissioner Roger Goodell, NFL lead negotiator Jeff Pash, and owners Jerry Jones of the Cowboys, Jerry Richardson of the Panthers and John Mara of the Giants emerged from Cohen's office shortly after 5 p.m., they sounded hopeful that negotiations would soon resume.
"We are prepared to come back here any time the union is ready to come back here," Pash said.