Many large companies maintain that they should not be required to bargain with employees of their contractors or franchisees, and that they should not be held liable for violations of those workers’ rights, if they exert control over the employees’ work conditions only in indirect ways, such as limiting what they can be paid for certain tasks.

The labor board explicitly rejected that logic. “It is not the goal of joint-employer law to guarantee the freedom of employers to insulate themselves from their legal responsibility to workers, while maintaining control of the workplace,” the Democratic majority wrote, addressing the purpose of the National Labor Relations Act. “Such an approach has no basis in the act or in federal labor policy.”

Wilma B. Liebman, a former N.L.R.B. chairwoman who wrote a crucial dissent in a 2002 case on the subject, said that the ruling was especially important because “sometimes the contractor is such a small entity, it exists on such a shoestring, that you have to get the lead firm to the table.”

The case the board ruled on involved a company called Browning-Ferris Industries of California, which the N.L.R.B. found was a joint employer of workers hired by a contractor to help staff the company’s recycling center. Unions are expected to seek to apply the ruling beyond the circle of companies that rely on contractors and staffing agencies, extending it to companies with large numbers of franchisees — even, some argue, to money managers who own significant stakes in corporations.

The joint employer designation could also make it easier to unionize in the first place. There have been instances in the past in which corporations appear to have terminated a franchise or contractor when that particular outfit was on the verge of unionizing, simply to avoid a union. This is legal for the corporation to do under existing law, but the franchisee or contractor cannot shut down on its own for this reason. As a joint employer, however, the corporation could no longer resort to this tactic.

“If you are a joint employer and you decide to shut down, there may well be liability,” Ms. Liebman said.

The ruling may have an immediate effect on a case the labor board is litigating against McDonald’s and several of its franchisees. In that case, the N.L.R.B.’s general counsel, who essentially acts as a prosecutor, asserts that the company is a joint employer along with a number of franchisees, making it potentially liable for numerous reported violations of workers’ rights, like retaliating against those who have tried to organize unions.