OTTAWA—The Supreme Court of Canada says an employer who wants to impose random alcohol testing on unionized workers in a dangerous work environment must show it is a reasonable move.

The court ruled that a mandatory random alcohol testing policy imposed by Irving Pulp and Paper at a Saint John, N.B., mill in 2006 was unreasonable and was properly rejected by a labour arbitration board.

In a 6-3 decision on Friday, the justices sided with the Communications, Energy and Paperworkers Union of Canada, which brought a grievance against the Irving policy.

A New Brunswick court overturned the arbitration board’s ruling against the company, but the Supreme Court restored it, saying the board was right to reject the tests because there was no evidence of an alcohol problem at the plant.

In 15 years before the policy was imposed, there were only eight instances in which a worker was found to be under the influence of alcohol and none involved an accident or injury. In the 22 months the policy was in effect, no one tested positive.

The justices said the applicable standard for judging such matters is reasonableness.

“In the end, the expected safety gains to the employer in this case were found by the board to range ‘from uncertain . . . to minimal at best’ while the impact on employee privacy was found to be much more severe,” Justice Rosalie Abella wrote for the majority.

The three dissenting judges said they found the arbitration ruling unreasonable and that it was properly set aside by the lower court.

Justices Marshall Rothstein and Michael Moldaver wrote the dissent, which was also supported by Chief Justice Beverley McLachlin.

They said the arbitration board wrongly balanced the competing interests of privacy and safety.