CHICAGO — The National Labor Relations Board on Monday blocked a historic bid by Northwestern University football players to form the nation’s first college athletes’ union, dealing a blow to a labor movement that could have transformed amateur sports.

In a unanimous decision, the board said the prospect of having both union and nonunion teams could lead to different standards at different schools — from the amount of money players receive to the amount of time they can practice — and create competitive imbalances on the field.

The ruling dismissed a stunning decision in March 2014 by a regional NLRB director in Chicago who said football players with scholarships are effectively school employees and entitled to organize. But Monday’s decision did not directly address the question of whether the players are employees.

The face of the union drive, former Northwestern quarterback Kain Colter, expressed disappointment in the ruling. But he said the unionization drive pressured the NCAA to take athletes’ grievances more seriously.

“Once we started this movement, within one year, things started changing,” he said. “It turned out to be the right thing to do, and I don’t regret it.”

Northwestern welcomed the ruling, and its vice president for university relations, Alan K. Cubbage, praised pro-union players, saying the school applauded them “for bringing national attention to these important issues.”

The labor dispute goes to the heart of American college sports, where universities and conferences reap billions of dollars, mostly through broadcast contracts, by relying on amateurs who are not paid. In other countries, college sports are small-time club affairs, while elite youth athletes often turn pro as teens.

The biggest factor in how it ruled, the board said, was the NLRB’s jurisdiction, which extends only to private schools like Northwestern, the sole private institution in the Big Ten. The board repeatedly cited the need for standardization of rules and policies in sports and said that giving the green light to just one team to collectively bargain would disrupt that uniformity.

“Processing a petition for the scholarship players at this single institution under the circumstances presented here would not promote stability in labor relations,” the ruling said.

NLRB rules do not offer the losing side a clear avenue to appeal Monday’s decision. But Ramogi Huma, a former linebacker at UCLA and a union activist who worked closely with Colter, said he has not given up on bringing unions to college football.

“The door’s not closed,” he said.

The board seemed to leave open the possibly of taking up the unionization issue again if it involved other schools or if conditions change for Northwestern football.

Northwestern became the focal point of the labor fight in January 2014, when Colter announced plans to form the first U.S. labor union for college athletes. He appeared at a news conference for the College Athletes Players Association, flanked by leaders of the United Steelworkers union, which lent its organizing expertise and presumably helped bankroll the union drive.

Three months later, regional NLRB Director Peter Sung Ohr issued his stunning decision, saying Northwestern football players who receive scholarships fit the definition of employees under federal law and therefore should be able to unionize. A month later, football players cast secret ballots on whether to unionize. Those ballots were sealed during the appeal and will now be destroyed without being counted.

While NLRB decisions are sometimes split along party lines, the three Democrats and two Republicans on the board all agreed.

Under U.S. law, an employee is regarded as someone who, among other things, receives compensation for a service and is under the direct control of managers. In Northwestern’s case, Ohr concluded that coaches are equivalent to business managers and scholarships are a form of pay.

The board’s decision was welcome news for the NCAA. It has been under increasing scrutiny over its amateurism rules and has been fighting lawsuits from former athletes over everything from head injuries to revenue earned from the use of their likenesses in video games.

In a statement, the Indianapolis-based NCAA portrayed the board’s ruling as recognition that it is trying to improve the lot of athletes.

“This ruling allows us to continue to make progress for the college athlete without risking the instability to college sports that the NLRB recognized might occur under the labor petition,” it said.

The NCAA recently cleared the way for the five biggest conferences, including the Big Ten, to add player stipends to help athletes defray some of their expenses. Southeastern Conference schools, for example, will give some athletes $3,000 to $5,500 each on top of a scholarship that pays for tuition, room, board and books.

Northwestern, the Big Ten and the NCAA all argued against the unionization effort, saying that lumping college athletes into the same category as factory workers would change amateur athletics for the worse. At one point, Northwestern administrators sent a document to players outlining potential pitfalls, noting that player strikes could lead to the spectacle of replacement players.

The specific goals of the players association, or CAPA, included guaranteeing coverage of sports-related medical expenses for current and former players and reducing head injuries.