PORTLAND — Maine's largest hospital is leaving the state's largest health insurer next year in a dispute over bill practices and delayed payments, its corporate parent announced Wednesday.

MaineHealth accused Anthem of underpaying and refusing to pay Maine Medical Center claims, while a spokesperson for Anthem accused MaineHealth of overcharging for its services.

Anthem owes MaineHealth in excess of $70 million for health care services dating back over three years, MaineHealth officials said.

Stephanie DuBois, spokesperson for Anthem Blue Cross and Blue Shield in Maine, accused MaineHealth of “unacceptable” fee increases.

Maine Gov. Janet Mills said Wednesday that she's “deeply concerned” and urged the two sides to resolve their differences. Anthem has 300,000 customers in Maine, she said.

“Termination of the contract would significantly harm the cost of and access to care for Maine people who are Anthem customers, particularly in southern Maine, and seriously impact the operation of the health care market across the state,” she said.

Anthem and MaineHealth have a contract that runs through Sept. 30, 2024, but MaineHealth intends to leave on Jan. 1.

The early announcement gives Anthem customers nine months to prepare, said Andrew Mueller, CEO of MaineHealth.

Both companies have engaged with an independent mediator but there has been “little progress,” he said.