A federal judge lifted the NFL’s lockout of its players Monday, ruling that they are suffering irreparable harm from the shutdown of the sport and that the owners’ action is improper because the players no longer are represented by a labor union.

U.S. District Judge Susan Richard Nelson granted a request by NFL players for a preliminary injunction that ends the lockout league owners imposed March 12, sweeping aside the league’s arguments about labor law and her authority to act.

But it’s not clear whether Nelson’s decision will bring the lockout to an immediate end. The NFL asked Nelson late Monday night for a stay of the injunction. It was not clear when she would rule on that. If she rejects it, the league would go to a federal appeals court.

Sources on both sides of the dispute said the players’ side believed Nelson’s ruling meant the free agent market technically was open Monday night, although the players’ side seemed willing to give the NFL a short amount of time to establish free agency rules. The league believed the free agent market would not open before a decision on a stay of the injunction. Several people on the players’ side of the dispute said they expected some players to show up for work Tuesday at teams’ facilities. Teams were advised by the league to allow players who show up to enter training facilities Tuesday, a source said.

The players “have made a strong showing that allowing the league to continue their ‘lockout’ is presently inflicting and will continue to inflict irreparable harm upon them, particularly when weighed against the lack of any real injury that would be imposed on the NFL by issuing the preliminary injunction. The public interest favors the enforcement of the antitrust laws and their underlying pro-competition policy,” Nelson wrote in her 89-page decision.

James Quinn, an attorney for the players, called Nelson’s ruling “a victory for the players and a victory for the fans. Hopefully we can get the players back to playing and the fans back to watching.”

Quinn also said in a telephone interview that “we’re obviously very pleased with the ruling. We’re not really surprised, because we think it’s consistent with all the prior case law and all the prior rulings. We think this will be very difficult to overturn on appeal because it’s very well reasoned and very well thought through.”

The NFL issued a written statement that said: “We will promptly seek a stay from Judge Nelson pending an expedited appeal to the Eighth Circuit Court of Appeals. We believe that federal law bars injunctions in labor disputes. We are confident that the Eighth Circuit will agree. But we also believe that this dispute will inevitably end with a collective bargaining agreement, which would be in the best interests of players, clubs and fans. We can reach a fair agreement only if we continue negotiations toward that goal.”

Under the lockout, players are barred from NFL facilities, teams are prohibited from signing or trading players and contact between coaches and players is precluded. The sport’s drug-testing program is not in effect and agents are unregulated.

If the league is unable to get a stay of the preliminary injunction and the lockout is lifted, the league would reopen for business and the upcoming season could be held while the litigation continues to play out in court. Sources previously said the league would be likely to use last season’s rules, which did not include a salary cap.

If the league gets a stay of the injunction, the sport’s first work stoppage in 24 years would continue indefinitely.

Some Washington Redskins players said they hadn’t received any instructions from the defunct players’ union, which dissolved itself on March 11, and didn’t know when or if they would. But linebacker Lorenzo Alexander initially said he planned to report to Redskins Park Tuesday morning unless a stay was granted by the appeals court.

“I’ve got a bonus due, so I’m planning on going and trying to work out and seeing if they let me in,” said Alexander, who is among players who would earn roster bonuses if they report for 85 percent of offseason workouts. “If they let me in, I’ll work out. If not, then I’ll go on my way.”

Alexander later changed his mind and said he would not go.

Other players, including Vonnie Holliday, who was the team’s union representative before decertification, said they presumed the lockout would still be in effect Tuesday.

Nelson’s decision came just more than five weeks after players filed an antitrust lawsuit against the owners on March 11 and 19 days after she told attorneys at the end of an April 6 hearing in St. Paul, Minn., that she would need a couple of weeks to rule on the players’ injunction request.

Nelson had ordered the league and the players to return to talks mediated by Chief Magistrate Judge Arthur J. Boylan. The two sides held four days of talks with Boylan at a federal courthouse in Minneapolis before recessing until May 16.

But Nelson cannot force them into an agreement and some legal experts have said they thought it unlikely the league and players would strike a deal before Nelson and the appeals court ruled. Some have said that the NFL believes it has a better chance to prevail in appellate court, which is known as a moderate-to-conservative panel of jurists.

In her lengthy ruling, Nelson addressed each of the league’s arguments about why she should not lift the lockout. She found the players’ decertification of their union to be valid and concluded that she did not have to wait for the National Labor Relations Board to complete an investigation of an unfair labor practice charge by the league against the union, as the NFL had urged her to do.

She also rejected an argument by the NFL that federal labor law, specifically the 1932 Norris-LaGuardia Act, prohibited her from granting the players’ injunction request in a labor dispute.

“The NFL . . . argues that the protections of labor law should apply for some indefinite period beyond the collapse and termination of the collective bargaining relationship. In the absence of either persuasive policy or authority, this Court takes a more conservative approach, and declines to do so,” Nelson wrote.

And she noted, the “public interest” does not favor the lockout.

“Professional football involves many layers of tangible economic impact, ranging from broadcast revenues down to concessions sales. And, of course, the public interest represented by the fans of professional football — who have a strong investment in the 2011 season — is an intangible interest that weighs against the lockout,” she added.

The lockout began one day after a previous round of negotiations overseen by federal mediator George H. Cohen collapsed March 11. The players dissolved their union that day and 10 plaintiffs — including high-profile quarterbacks Tom Brady, Peyton Manning and Drew Brees — filed the antitrust lawsuit against the owners. That case was assigned to Nelson, who later combined it with a separate lawsuit filed by former players. The NFL draft is set to take place Thursday through Saturday as scheduled under a provision in the now-expired collective bargaining agreement.

Staff writer Mike Jones contributed to this report.