The United Food and Commercial Workers' complaint comes after The Wall Street Journal reported last week that former Wal-Mart Vice Chairman Tom Coughlin may have used undocumented expense payments to fund anti-union activities, including paying union staffers to tell him of pro-union workers in stores.

The union said its complaint, filed on Tuesday, asks the NLRB to "aggressively investigate whether Wal-Mart bribed employees to suppress worker support for union representation."

Shares of Wal-Mart, which have fallen about 6 percent in the past month, edged up 0.5 percent after Prudential Equity Group raised its rating on the stock to "neutral weight" from "underweight."

"Wal-Mart's actions seemingly involved the criminal misappropriation of company funds to create an illegal anti-union slush fund," the union said in a statement.

The union wants the NLRB to subpoena any documents from Wal-Mart that might substantiate those charges.

A Wal-Mart spokeswoman did not immediately return a call seeking comment. The retailer has previously said it investigated the allegation of payments to union representatives and found no evidence to support it.

In a letter to the NLRB accompanying the complaint, the union said it suspected that Wal-Mart "spread bribes in stores whose workers were actively organizing but abruptly abandoned their activity" in 13 U.S. states.

Wal-Mart is the largest U.S. private employer with more than 1.2 million employees. The company has repeatedly said its policy of open communication with employees means there is no need for a union, but labor groups contend the retailer is vehemently anti-union.

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