WASHINGTON — Most private employers would have to display posters informing workers about their right to form a union under a proposed federal rule that is bound to please unions and draw the ire of companies trying to resist labor organizers.

The planned rule, announced Tuesday by the National Labor Relations Board, would require businesses to post notices in employee break rooms or other prominent locations to explain a worker's rights to bargain collectively, distribute union literature or engage in other union activities without reprisal.

The move to issue a broad rule signals a more aggressive posture by the labor board, which usually makes policy on a case-by-case basis in individual labor-management disputes.

It comes less than a year after President Barack Obama made several recess appointments to give the board its first Democratic majority in a decade. Obama's appointments to the board were held up for months over GOP concerns that one nominee — former AFL-CIO counsel Craig Becker — would be too sympathetic to unions.

As unions struggle to pass pro-labor legislation in Congress, leaders are increasingly looking to the labor board and other federal agencies to help reverse what they view as an increasingly hostile atmosphere for organizing new members.

Unions are trying to reverse years of membership declines in the private sector, where just 7.2 percent of employees belong to a union.

In recent months, the labor board has expressed an interest in allowing electronic voting for union elections, a move that business groups fear could compromise secret ballots. The board is also speeding up its review of cases where employees are fired during union organizing drives.

The rule would not take effect for at least 60 days, during which the agency is taking comments.

In a statement, the board said it believes many employees protected by the National Labor Relations Act "are unaware of their rights" under the law. The notices make clear that workers don't have to join a union and outline other legal protections against union intimidation or misconduct.

Similar posters are already required to be displayed in the offices of government contractors and subcontractors under a White House executive order that took effect in June. The directive was one of the first executive orders Obama signed shortly after taking office last year.

Michael Lotito, a San Francisco labor attorney who represents employers, said the notices could make it appear that the government is sanctioning unions.

"It will stimulate interest and curiosity among employees as to why this happened, which will prompt them to make telephone calls to unions," Lotito said.

Michael Eastman, executive director of labor policy at the U.S. Chamber of Commerce, questioned whether the board even has the legal authority to issue the rule. If it does go forward, he suggested the posters should be more balanced for those who don't want to join unions.

The AFL-CIO did not immediately respond to a request for comment.