(Bloomberg) -- The United Auto Workers is asking the federal labor board to investigate Tesla Inc. over a tweet by Elon Musk.

In a complaint filed Wednesday and obtained by Bloomberg News, the union alleges that the electric-car maker violated the National Labor Relations Act by threatening to take away employee stock options in retaliation for engaging in protected union activity.

The union’s filing cites a statement that Tesla’s Chief Executive Officer Musk made this week on Twitter: “Nothing stopping Tesla team at our car plant from voting union. Could do so tmrw if they wanted. But why pay union dues & give up stock options for nothing?”

The UAW, which is actively trying to organize Tesla’s Fremont, California, assembly plant, notes in its complaint that Musk has 21.8 million Twitter followers, that the tweet was widely shared and covered in the media and that it remains online.

A Tesla spokesman said Musk’s tweet was meant to reflect that UAW members working for other automakers don’t get stock options or restricted stock units. Tesla’s compensation is the highest in the industry because of the options, but UAW organizers dismiss the value of the equity, according to the company.

Former NLRB Chairwoman Wilma Liebman, who has done legal work in the past for the UAW, told Bloomberg News on Tuesday that Musk’s tweet could be grounds for an NLRB complaint. “The employee is going is going to hear it as, ‘If I vote to unionize, stock options will no longer be an option,’” she said.

Federal labor law prohibits employers from threatening to retaliate against employees for organizing.

--With assistance from David Welch and Dana Hull.

To contact the reporter on this story: Josh Eidelson in Washington at jeidelson@bloomberg.net

To contact the editors responsible for this story: Craig Trudell at ctrudell1@bloomberg.net, Jamie Butters

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