Athletes at private universities should be considered employees under federal labor law, and therefore allowed to unionize and seek protection from retaliation if they voice concerns about pay and workplace conditions, according to the top lawyer for the federal National Labor Relations Board.

Jennifer A. Abruzzo, the N.L.R.B.’s general counsel, also warned in a memo released Wednesday that universities “misclassifying such employees as mere ‘student-athletes’” could be threatened with legal action for creating a “chilling effect” on athletes who wanted to organize.

The memo represents guidance to the N.L.R.B.’s regional offices and other labor officials, and is therefore not binding. Moreover, the board’s jurisdiction covers private universities, not public ones, which are governed by state labor laws and agencies. Public universities dominate the collegiate athletic world.

Still, academics, as well as lawyers who have represented both athletes and universities, concurred that the new N.L.R.B. under the Biden administration had sent an unmistakable message that it plans to be aggressive in the area of college sports — and by extension, against the N.C.A.A.

In an interview after the memo was released, Abruzzo said: “I’m hoping to educate. I’m hoping to deter violations. I’m hoping to ensure that the players and others know what their rights are. It is guidance for everyone, and it is frankly putting players and workers and their employing institutions on notice.”

“With college sports embedded within the higher education experience, we firmly believe that college athletes are students who compete against other students, not employees who compete against other employees,” the N.C.A.A. said in a statement.

But Joseph Ambash, a partner at Fisher Phillips, which represents employers in labor law, said the memo was tantamount to “a D.A. saying these are the cases I’m going to prosecute.”

“For employers, this is telling is that there’s going to be a lot more litigation and a lot more vigorous enforcement of the N.L.R.A. against employers,” said Ambash, who represented Brown University in a landmark case that found that graduate students were not employees under the National Labor Relations Act.

“There’s a complete swing: under the Trump board there were many, many, many decisions that were favorable to employers,” Ambash continued. “Under the Biden board they may reverse some decisions and make them more friendly to labor.”

In the memo, Abruzzo noted recent “significant developments in the law, NCAA regulations, and the societal landscape” demonstrated that the traditional idea of amateur sports had changed irrevocably.

She cited the Supreme Court’s unanimous ruling, in N.C.A.A. v Alston, which asserted that college sports was a profitable enterprise. She mentioned the N.C.A.A.’s decision, in the face of mounting pressure being exerted by state legislatures, allowing athletes to make money from their fame under new name, image and likeness laws. And she noted how athletes had been “engaging in collective action at unprecedented levels,” like demanding social justice after George Floyd’s murder and insisting that their health and safety be factored into any decisions related to playing during the coronavirus pandemic.

The confluence of these events also comes at a far different time, Abruzzo noted, than the last time the N.L.R.B. weighed in on college athletes in a high-profile fashion.

In 2015, the five-member board — overriding the decision of a regional director — ruled against football players at Northwestern, a private school, who were seeking to unionize. The board did not rule directly on the core question of whether the players were university employees, but rather said that the petition’s impact on sports would not have promoted “stability in labor relations.”

“They punted,” said Wilma Liebman, the chair of the N.L.R.B. from 2009 to 2011. “But win, lose or draw, the athletes really won because there was so much attention paid to them, and so much sympathy.”

Under the new guidance, Liebman and others cautioned, it could take months, or even years, for the right case to bubble up through the N.L.R.B. system.

“We are a long way from college athletes being employees,” said Gabriel Feldman, a law professor who is the director of the Tulane Sports Law Program. “But we are certainly at a point where we have indications from many external authorities who believe that the status quo with respect to college athletic rights is not sustainable.”

He added: “If you think of external threats to the N.C.A.A., this is another external threat.”

The first public opportunity for college sports leaders to discuss the memo with lawmakers could come on Thursday.

That is when a House subcommittee is scheduled to hold a hearing on college athletes’ rights. Mark Emmert, the president of the N.C.A.A., is scheduled to testify. So is Ramogi Huma, the executive director of the National College Players Association, who was active in the Northwestern case.

“I will be arguing for broad-based reform,” Huma said.

Billy Witz and Noam Scheiber contributed reporting.