The Michigan Court of Appeals last week upheld a magistrate’s 2011 ruling which found that General Motors Co. had improperly reduced workers compensation benefits to some retired workers.

In an unpublished decision, the three-judge panel ruled that a 2009 deal struck between the United Auto Workers and a financially distressed GM to coordinate pension and workers compensation benefits was improper.

The now deceased plaintiff in the case, Clifton M. Arbuckle, began working at GM in 1969 and was awarded workers compensation benefits in 1995 at a fixed rate of $362.78 a week. In November 2009, General Motors sent Mr. Arbuckle a letter informing him that as of Jan. 1, 2010, his benefits would be reduced to $262.55 per week in light of the coordination agreement struck with the UAW.

In March of 2011, Kenneth A. Birch of the Michigan Workers’ Compensation Board of Magistrates issued a ruling that found that GM had improperly reduced Mr. Arbuckle’s benefits, noting that there was insufficient evidence to establish that the UAW had authority to bargain on his behalf and bind him to the 2009 agreement.

Mr. Arbuckle passed in April 2014.

GM appealed Mr. Birch’s ruling and in 2012 the Michigan Compensation Appellate Commission reversed the magistrate’s decision.

Last week’s ruling by the Michigan Court of Appeals reverses the compensation appellate commission’s decision. In their ruling, the judges said the union did not have the authority under federal labor law to change the collective bargaining rights of the retirees.

“It is simply not tenable that a contract could be amended with respect to a particular party when that party had no representation during the amendment process,” the ruling states. “Here, there was no ‘bargaining’ between plaintiff and the UAW with regard to the allowance of coordination.”

A spokesman for GM said the company will appeal the decision. “We are disappointed by the decision and intend to appeal,” the spokesman said.

The attorney representing the Arbuckle estate in the case, Flint, Michigan-based Robert J. MacDonald of MacDonald & MacDonald P.L.L.C., said the ruling should help the hundreds of retirees that saw reductions to their benefits. “I’m really happy that we prevailed, it’s a nice victory for these workers,” Mr. MacDonald said. “I certainly hope this case takes care of those who were promised unreduced workers compensation benefits under GM/UAW pensions and contracts.”