A US labour official has recommended the results be nullified in a failed vote to unionise Amazon workers at an Alabama warehouse, the union in the effort said on Monday, opening a possible path to a new election.

The recommendation by a hearing officer is a key step in potentially overturning the April ballot, which aimed to create the first union at US-based Amazon facility but which the union alleges was tainted by the company's interference.

US labour watchdog, National Labour Relations Board (NLRB), would need to give its approval for the proposal to take effect. 

The results, showed a wide majority of workers rejecting the move, capped a bruising months-long battle that sparked intense debate over workplace conditions at Amazon, which has more than 800,000 US employees.

"We support the hearing officer's recommendation that the NLRB set aside the election results and direct a new election," Retail, Wholesale and Department Store Union President Stuart Appelbaum said in a release.

"Amazon's behavior throughout the election process was despicable."

Amazon has held firm that it did not interfere with the voting, and said it will appeal the hearing officer's recommendation.

"Our employees had a chance to be heard during a noisy time when all types of voices were weighing into the national debate, and at the end of the day, they voted overwhelmingly in favor of a direct connection with their managers and the company," Amazon said in response to an AFP inquiry.

"Their voice should be heard above all else, and we plan to appeal to ensure that happens."

Amazon has argued that most of its workers don't want or need a union and that it already provides more than most other employers, with a minimum $15 hourly wage and other benefits.

The labour group said workers were bombarded with anti-union messages, and claimed the company's use of a drop-box outside the warehouse could have intimidated employees.

The Amazon drive was seen as a watershed for a diminished US labour movement, with activists aiming to use the Alabama warehouse as a catalyst for other organising effort