A hearing officer who handled Amazon’s appeal of the union’s victory concluded that the company “has not met its burden” to prove that the union, the government or anyone else “engaged in objectionable conduct affecting the results of the election”, National Labour Relations Board spokesperson Kayla Blado said in an email.

Amazon spokesperson Kelly Nantel said the company was still reviewing the decision, but “we strongly disagree with the conclusion and intend to appeal. As we showed throughout the hearing with dozens of witnesses and hundreds of pages of documents, both the NLRB and the ALU improperly influenced the outcome of the election and we don’t believe it represents what the majority of our team wants”.

The Seattle-based company, which had managed to keep unions out of its US operations for more than a quarter-century, had argued in a filing that the labour board repeatedly “failed to protect the integrity and neutrality of its procedures”.

Amazon has until 16 September to file objections to the hearing officer’s recommendation, which will then be heard by a regional director of the agency. If the company fails to persuade the agency to overturn the vote results, it will be legally required to negotiate with the union over pay and working conditions at the Staten Island warehouse. Employers sometimes refuse to negotiate even after exhausting their appeals at the labour board, leading to lengthy litigation in federal court. The NLRB lacks the power to impose punitive damages on companies for noncompliance.

The victory in April in New York by the upstart ALU, led by fired employee Christian Smalls, is among the most remarkable in a series of landmark labour wins over the past year, which include successful union elections at Starbucks, Trader Joe’s and Chipotle Mexican Grill.

“To have a contract for Amazon workers in this country would be monumental for the labour movement,” Smalls said on Thursday. “We always were confident that we beat them fair and square.” BM