Admitting at least temporary defeat, the National Labor Relations Board missed a deadline to appeal the rejection of its so-called "poster rule" to the Supreme Court. The federal labor law enforcement agency will still be able to try reinstate the rule next year, but will have to start the rulemaking process from scratch. That would take several months at least.

The rule, which the NLRB adopted in 2011, would have required all businesses to prominently place posters in the workplace explaining workers' rights to join a union.

Failure to do so would have been evidence of a hostile work environment toward union activities, the NLRB said. Those posters would not include other information though, such as how to decertify a union or how workers can opt out of paying dues.

Employers have pushed back and won several court battles. The U.S. Circuit Court of Appeals for the District of Columbia first vacated the NLRB's ruling in May, saying it violated the First Amendment. In June, the Fourth Circuit Court of Appeals also ruled against the board, saying it exceeded its authority under the National Labor Relations Act.

On Friday, the NLRB did not file a challenge to the court's rulings. Critics of the rule said President Obama's NLRB appointees were signaling that they knew they could not win on appeal.

"It appears the unanimous voice of the judiciary has forced the Obama labor board to back down from its attempt to empower union bosses yet again at the expense of the rights of employees and employers," said Mark Mix, president of the pro-business National Right to Work Foundation.

A spokesman for the NLRB could not be reached for comment.