Body armour manufacturer DHB Industries Inc is facing an official complaint from workplace watchdogs amid claims it sacked three workers who planned to join a union.

The National Labour Relations Board (NLRB) said it plans to file a complaint against the firm following an eight-week probe at its Oakland Park factory in Florida. Investigators allege the workers were fired for retaliatory reasons.

Union chiefs are also calling for a probe into working conditions at the plant that they say includes limited access to drinking water, high temperatures, two toilets for almost 400 employees, one sick day per year and limited health-care benefits.

DHB described the complaint as "unfounded" and said it was confident of a successful outcome to any NLRB action.