In a decision that could affect millions of workers across the country, the National Labor Relations Board ruled on Thursday that employers could not prohibit employees from using their company’s email to communicate and engage in union organizing on their own time.

The 3-to-2 ruling overturned a decision made in 2007, when Republicans held a majority on the board, that had forbidden such use of email.

Calling that ruling “clearly incorrect,” the current majority noted how technology had transformed daily habits. “The workplace is ‘uniquely appropriate’ and ‘the natural gathering place’ for such communications,” the board wrote, “and the use of email as a common form of workplace communication has expanded dramatically in recent years.”

Image John B. Langel, a lawyer at the firm Ballard Spahr, said the major problem with the decision is that it is impossible for an employer to monitor whether people are opening, reading and forwarding the emails when they are supposed to be working. Credit... Ballard Spahr

The board did carve out an exception, saying that in special circumstances, employers might be able to create an overall ban on nonwork use of email if they could show it was necessary for productivity or discipline. The board said that as long as workers were allowed to send non-work-related emails, then employers could not bar the messages from being about union organizing.