SEATTLE — An appeals court has upheld the largest penalty for campaign finance violations ever assessed in Washington state.

In a unanimous ruling Tuesday, the court affirmed the $18 million fine imposed on the Grocery Manufacturers Association for violating Washington campaign-finance laws during a 2013 fight against a food-labeling initiative, The Seattle Times reported.

The Washington State Court of Appeals found the Grocery Manufacturers Association’s violations “serious and significant” and “represented an intentional attempt to conceal the identity of companies donating millions of dollars in a contentious ballot campaign.”

The case stemmed from Initiative 522, which would have required the labeling of genetically modified organisms, or GMOs, in food products sold in Washington. Voters narrowly rejected the measure, with a record $22 million spent by the “no” campaign.

The Grocery Manufacturers Association, which has changed its name to the Consumer Brands Association, spent over $11  million to defeat it. But its contributions were disclosed only as coming from the association, not the companies that bankrolled the effort, including PepsiCo, Coca-Cola, Nestle and General Mills.

Washington Attorney General Bob Ferguson sued, saying the group had failed to properly register as a political committee and flouted transparency laws. His attorneys produced internal association documents showing the trade group had sought to insulate individual companies from public blowback by hiding their names.

After the lawsuit was filed, the association agreed to disclose the donors. But Ferguson’s office argued the damage was already done, with the information hidden for much of the 2013 campaign.

“Dark money has no place in Washington elections,” Ferguson said Tuesday in a statement. “This decision confirms that our courts take intentional violations of our campaign finance laws seriously. My office will continue to stand up for Washingtonians’ right to know who is influencing our elections.”

The grocery group’s attorneys had argued the state’s penalties were excessive and a violation of its constitutional rights.

On Wednesday, the Consumer Brands Association said in a statement it was “eager to close the door on one of the last legacy issues of the Grocery Manufacturers Association and put this case behind us.”

The statement added: “The ruling is clearly disproportional to other campaign finance violations of its kind, and we will continue to exhaust a series of legal options.”

Tuesday’s appeals court ruling confirmed previous decisions in the long-running case.