A federal appeals court on Tuesday struck down a National Labor Relations Board rule requiring most private sector employers to post a notice informing employees of their right to unionize.

Ever since the labor board proposed the rule in December 2010, business groups have asserted that the move exceeded the board’s authority and was an improper imposition on nearly six million employers, most of them small businesses.

In its decision, the United States Court of Appeals for the District of Columbia Circuit concluded that the N.L.R.B.’s rule violated a federal law that bars the board from punishing an employer for expressing its views so long as those statements do not constitute threats of retaliation or force.

The labor board had originally said that an employer’s failure to post the notice would be considered an unfair labor practice, resulting in penalties, but the circuit court said the board would be acting illegally to punish an employer for expressing a statement or in this case, for failing to post a statement under orders by the labor board.