Remarks that Jassy made to reporters about the downsides of unionisation told “employees that, if they selected a union, they would become less empowered and would find it harder to get things done quickly”, NLRB administrative law judge Brian Gee wrote.

Gee cited various comments Jassy made, including telling CNBC that making workplace improvements is “much slower” with a union and saying at a New York Times conference that employees without a union are “better off” because “it’s not bureaucratic”.

Amazon CEO Andy Jassy has made comments about unions that a US National Labor Relations Board judge said violated federal labour law.

Gee also cited Jassy for telling the Bloomberg Technology Summit that in a union shop, “if you see something on the line that you think could be better for your team or you or your customers, you can’t just go to your manager and say, ‘Let’s change it’.”

While precedent establishes that a manager can make factually based predictions about “demonstrably probable consequences beyond his control,” Gee wrote that Jassy “offered no objective basis” for his assertions.

The judge said Amazon should be forced to post a notice at its US facilities informing employees of their rights and committing not to threaten them.

Amazon said it strongly disagrees with the ruling and plans to appeal. “The decision reflects poorly on the state of free speech rights today, and we remain optimistic that we will be able to continue to engage in a reasonable discussion on these issues where all perspectives have an opportunity to be heard,” spokesperson Mary Kate Paradis said in an email.

The Amazon Labor Union, which brought the case to the NLRB, celebrated the ruling. “It sends a clear message that attempts to dissuade workers from exercising their right to organise and bargain collectively will not be tolerated,” the union’s attorney, Seth Goldstein, said in a text message.

US law allows companies to oppose and argue against unionisation, but not to threaten to punish workers if they organise.

Rulings by NLRB judges can be appealed to labour board members in Washington, and from there into federal court. The agency lacks the authority to hold executives personally liable for violations or make companies pay punitive damages.