BOSTON — Efforts by the biggest health care system in Massachusetts to become even bigger have received another challenge, this time by a key state board.

The Health Policy Commission found Wednesday that Partners HealthCare’s proposed takeover of Hallmark Health System, which runs two community hospitals north of Boston, would reinforce Partners’ market power, raise spending on medical care by up to $23 million per year, and increase premiums for employers and consumers.

“It is unclear how this merger is necessary to improve clinical quality in ways the parties’ longstanding affiliation has not,” the commission’s report said, according to The Boston Globe.

It asked Partners and Hallmark to submit more information.

Partners operates Brigham and Women’s and Massachusetts General hospitals, as well as several community hospitals, health centers, and a health plan. Its plans for Lawrence Memorial Hospital in Medford and Melrose-Wakefield Hospital in Melrose include converting the Medford hospital to a short-stay facility, while renovating the Melrose facility.

The Health Policy Commission also had concerns about Partners’ bid to acquire South Shore Hospital in Weymouth.

Attorney General Martha Coakley brokered a deal with Partners that would allow it to complete the acquisition of South Shore under conditions that, in part, are meant to control costs. A judge is considering the deal.

Partners’ competitors are also opposed to the deal.