WASHINGTON — In a victory for unions, the National Labor Relations Board ruled Thursday employees can use their company e-mail accounts for union organizing and other workplace-related purposes, if they do it on their own time.

Once an employer gives an employee access to the company e-mail system, then the business cannot restrict what the employee e-mails, so long as it is generally workplace-related and isn’t during working hours, the NLRB ruled. The NLRB is a government agency that investigates unfair labor practices.

The ruling said “the use of e-mail as a common form of workplace communications has expanded dramatically in recent years.” The ruling could give unions a powerful organizing weapon.

The three Democrats on the five member board voted “yes,” while the two Republicans abstained.

The ruling reverses a 2007 board decision that employees don’t have a legal right to use their employers’ e-mail for union activity or discussing wages or other workplace issues.

It also upholds an opinion by the NLRB’s general counsel, who suggested workers had a presumed statutory right to use company e-mail to discuss a range of workplace issues — so long as they did it on their own time.