US labor authorities on Monday ordered a new vote on whether Amazon warehouse workers in Alabama can form a union, saying the online retail giant had violated rules during the ballot.

Organizers vying to form the first union at the e-commerce colossus in the United States had appealed after employees voted overwhelmingly in April against the effort.

No new date was set and Amazon declined to say in an email to AFP whether it plans to appeal the order from a National Labor Relations Board (NLRB) official overseeing the matter.

"Today's decision confirms what we were saying all along -- that Amazon's intimidation and interference prevented workers from having a fair say in whether they wanted a union in their workplace," said Stuart Appelbaum, president of the Retail, Wholesale and Department Store Union.

Regional NLRB official Lisa Henderson, in her decision, cited objections from the organizers, such as Amazon creating the "impression that it was recording the identity of employees who voted."

Amazon has been ordered to re-do US union vote Amazon has been ordered to re-do US union vote Photo: AFP / Marco Bertorello

Amazon, in a statement, said workers at the site had already rejected forming a union.

"It's disappointing that the NLRB has now decided that those votes shouldn't count. As a company, we don't think unions are the best answer for our employees," wrote Kelly Nantel, an Amazon spokesperson.

The vote in the small Alabama town of Bessemer was the focus of much attention, with the vote pitting supporters of the unionized employees -- artists, Democratic and Republican lawmakers and even President Joe Biden -- against Amazon, whose business has flourished during the Covid-19 pandemic.

Amazon is currently facing a group of workers in a New York warehouse also seeking to create a union.

The NLRB's decision also comes at a time when another major US company, the Starbucks coffee chain, is also engaged in a bitter struggle over unionization at three coffee shops in the northeast United States.

Management has deployed significant resources to convince employees at these three locations to vote against forming a union. They have until December 8 to return their ballots.