By Daniel Wiessner

(Reuters) - U.S. labor board prosecutors have determined that a union's claims that Chipotle Mexican Grill (CMG) illegally refused to give raises to workers at a Michigan restaurant after they became the first and only employees of the fast-casual chain to unionize have merit.

The National Labor Relations Board's general counsel will issue a formal complaint against Chipotle unless it settles the claims by the International Brotherhood of Teamsters, NLRB spokeswoman Kayla Blado said on Monday.

Workers at the Lansing, Michigan, restaurant voted 11-3 to join the Teamsters in 2022, but have yet to secure a contract with Chipotle. The union in a complaint filed last year said Chipotle withheld raises from the store's employees by falsely claiming that they were not eligible for pay bumps because they had unionized.

Chipotle and lawyers for the Teamsters did not immediately respond to requests for comment.

The company last year agreed to pay $240,000 to settle a separate NLRB case claiming it illegally shuttered an Augusta, Maine, restaurant in response to a union campaign there. Chipotle denied wrongdoing but also agreed to post notices about workers' legal rights at 40 stores throughout New England.

If the general counsel issues a complaint in the Michigan case, it will be heard first by an administrative judge and then by the five-member board appointed by the president, whose decisions can be appealed to federal appeals courts.

The NLRB is facing a series of lawsuits, including at least five filed this month, claiming that its in-house enforcement proceedings violate the U.S. Constitution.

Several companies, including rocket maker SpaceX and pipeline operator Energy Transfer, have sued the agency after being hit with complaints alleging illegal labor practices.

(Reporting by Daniel Wiessner in Albany, New York, Editing by Alexia Garamfalvi and Bill Berkrot)