WASHINGTON — The Supreme Court announced Tuesday that it will not decide whether a type of agreement between employers and unions that has become increasingly important to the labor movement violates the law.

The court dismissed as “improvidently granted” a case it heard last month that both unions and right-to-work groups had called one of the major labor issues of the term. The terminology means that justices found a procedural flaw in the case that kept them from deciding it.

The case was about “neutrality agreements.” In such accommodations, an employer might remain neutral during a union-organizing campaign and even grant access to company grounds or lists of employees. In return, the union might agree to give up the right to strike or throw its support behind a matter important to the company.

The agreements have long been used and have been found lawful by several federal appeals courts.

But in the case of a union seeking to organize workers at a Florida greyhound track and casino, the U.S. Court of Appeals for the 11th Circuit found that the agreement violated a section of the Labor Management Relations Act. The law forbids an employer to “pay, lend, or deliver … any money or other thing of value” to a labor union seeking to organize the company’s workers.

Mardi Gras Gaming allowed the union, Unite Here, access to its employees and agreed to election rules allowing employees to vote by checking a card in front of others rather than by secret ballot.

The union made concessions as well and agreed to spend $100,000 to support a 2006 referendum to allow slot machines at the casino.

Justice Stephen Breyer dissented from the court’s action, saying it should not have dismissed the case without further action.

He acknowledged that it was possible the Florida case is moot because the contract between Mardi Gras Gaming and Unite Here expired at the end of 2011, before the 11th Circuit rendered its decision. But Breyer said he would call for more briefing on that issue.

If the case is moot, he said, the court should likely vacate the 11th Circuit’s decision.

“Unless resolved, the differences among the courts of appeals could negatively affect the collective-bargaining process,” Breyer wrote.

“This is because the Eleventh Circuit’s decision raises the specter that an employer or union official could be found guilty of a crime that carries a 5-year maximum sentence … if the employer or union official is found to have made certain commonplace organizing assistance agreements with the intent to ‘corrupt’ or ‘extort.’ ”

He was joined in the dissent by Justices Sonia Sotomayor and Elena Kagan.

The case is Unite Here Local 355 v. Mulhall.