BOSTON — Massachusetts State Police must reinstate seven troopers who refused to be vaccinated for COVID-19, an independent arbitrator has ruled. The troopers have been on unpaid leave, but the arbitrator’s decision means they can return to work with retroactive pay if they choose.

The union representing state troopers, which plans to hold a news conference Monday outside the State House, filed a grievance after the law enforcement officers were suspended following former Republican Gov. Charlie Baker’s 2021 order requiring executive department employees to be vaccinated. Current Democratic Gov. Maura Healey lifted the vaccine mandate in May.

Massachusetts State Police are in the process of determining the “scope as well as the administrative and legal steps” needed to implement the arbitrator’s ruling, David Procopio, an agency spokesperson, said Sunday in an email.

The arbitrator concluded State Police violated a collective bargaining agreement in the way they handled the cases of eight troopers who cited religious grounds for refusing to take the vaccine. The agency summarily dismissed the troopers instead of reviewing their accommodation requests, the arbitrator said Friday. One of the eight troopers later returned to work.

The State Police Association of Massachusetts criticized the former Baker administration for refusing to work with the troopers.

“These members, whose religious convictions were trampled, and who were left without pay or benefits, now can choose to return to work and will be made whole through retroactive pay and earned seniority,” said Patrick McNamara, the union president.

He said the union will continue to fight for another 13 troopers who weren’t deemed eligible for exemptions and were fired or dishonorably discharged for failing to get vaccinated.