The National Labor Relations Board is postponing the effective date of a new rule requiring most private businesses to put up a poster that tells workers about their right to form a union.

The board said Wednesday there has been so much confusion about which businesses are covered under the rule that officials want to conduct more outreach, especially to small and medium sized businesses.

"We got a lot of calls from various businesses that are just not familiar with this law and are not aware they even fall under our jurisdiction," said board spokeswoman Nancy Cleeland.

The rule was supposed to take effect on Nov. 14, but that date has now been moved to Jan. 31. It requires nearly every private business to post the 11-by-17-inch notice in a prominent location explaining a worker's right to bargain collectively, distribute union literature and engage in other union activities without reprisal.

The poster includes language explaining the legal right not to join a union.

"We realize we need to do a lot more outreach to let businesses know that the National Labor Relations Act covers employers whether or not they have a unionized work force," Cleeland said.

The rule produced outrage in the business community and at least three major lawsuits challenging the board's authority to require companies to put up the poster.

Businesses groups trying to block the rule _ the U.S. Chamber of Commerce, the National Association of Manufacturers and the National Federation of Independent Business are among them _ claim the posters are a government effort to encourage workers to unionize.

Failure to put up the posters under the new rule would be considered an unfair labor practice. But Cleeland said the board is not trying to implement a "gotcha" rule and would probably issue a warning first if there is a complaint about a company's failure to comply.

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