The National Labor Relations Board has denied Station Casinos’ appeal of a regional director’s decision to approve the results of a unionization election at one of its properties in November.

Station Casinos has disputed the results of the unionization election at Green Valley Ranch in Henderson for months, filing a complaint alleging unfair campaign practices by the Culinary Union after the November vote and submitting a final appeal to the National Labor Relations Board in Washington D.C. this spring to review a regional director’s decision to certify the election results. The board on Thursday denied the review, saying that Station Casinos’ request “raised no substantial issues warranting review” and that the union could not have “reasonably tended to interfere with employees’ free choice in the election.”

The board wrote in its order that Station Casinos had failed to prove that any of the employees of Green Valley Ranch could have known or “reasonably inferred” that the union had made a list of employees who had not yet voted in the election. The board also said that all of the union’s actions were in response to information employees voluntarily provided to it and therefore “could not reasonably give rise to an impression of surveillance.”

Workers at Green Valley Ranch voted 78 percent in favor of unionization during the two-day election on Nov. 8 and 9. About 900 workers will be covered by the Culinary Workers Union Local 226 and Bartenders Union Local 165.

In the wake of the order, the union is calling on Station Casinos to begin contract negotiations immediately, saying that if it does not do so it will be in violation of federal law. The union’s secretary-treasurer Geoconda Arguello-Kline said in a statement that she was “disappointed” the company decided to litigate its challenges.

“We urge Station Casinos to recognize their workers’ voices and votes at Green Valley Ranch and the Palms, and begin contract negotiations in good faith immediately,” Arguello-Kline said.

A spokeswoman for Station Casinos did not immediately respond to a request for comment.

Green Valley Ranch is the fourth Station Casinos property to unionize, with two other Las Vegas properties, Boulder Station and Palace Station, also in the labor fold. Workers at the Palms voted 84 percent in April to unionize as well, though Station Casinos has asked the National Labor Relations Board to review the regional director’s decision to hold the election after the company had asked the election to be delayed until 2019.