Hundreds of Amazon employees Sunday openly criticized the online retail giant’s environmental record, defying the company’s communications policy.

More than 300 signed a Medium blog post by Amazon Employees for Climate Justice (AECJ), which is pushing the company to go further in its climate change mitigation plan announced in September.

Group members have publicly criticized the company, and some have been warned that they could be fired.

“The protest is the largest action by employees since Amazon began threatening to fire workers for speaking out about Amazon’s role in the climate crisis,” the AECJ said.

“As Amazon workers, we are responsible for not only the success of the company, but its impact as well,” said Sarah Tracy, a software development engineer at Amazon.

“It’s our moral responsibility to speak up, and the changes to the communications policy are censoring us from exercising that responsibility.”

It is common for companies to demand restraint from employees when it comes to publicly discussing the firm’s activities and even more so when openly questioning them.

While the environment and climate change was the focus of many of the posts on Sunday, Amazon was also criticized for other activities, such as providing artificial intelligence capabilities to companies in the oil sector.

Amazon, which in December said its workforce had hit 750,000, is often criticized over its carbon footprint because of the high energy consumption of its huge server farms for its lucrative cloud computing activities.

And it has built its success on the back of a huge road transport logistics network to ensure speedy deliveries, which generates a lot of greenhouse gases, the main culprit in climate change.

Amazon founder Jeff Bezos on Sept. 19 last year made public environmental commitments, promising in particular that the firm would be carbon neutral by 2040.

The AECJ said this was insufficient and that Amazon should be aiming for a 2030 target.

“This is not the time for silencing voices. We need policies that welcome more open discourse, more problem-solving, and more urgent and concerted action about climate change and its causes,” said Mark Hiew, a senior marketing manager at Amazon.

Responding to the letter, an Amazon spokesperson defended its policy on public statements about the company.

“While all employees are welcome to engage constructively with any of the many teams inside Amazon that work on sustainability and other topics, we do enforce our external communications policy and will not allow employees to publicly disparage or misrepresent the company or the hard work of their colleagues who are developing solutions to these hard problems,” the company said.