A ballot of GMB members at the airport could see a walkout “at the end of August”, according to the union.
The airport told the BBC fewer than 25% of staff were threatening possible action, claiming the strike vote was "without warning and without discussions".
The GMB said many staff “were struggling on the minimum wage” and faced increasing work pressure due to chronic under-staffing.
The union said a discretionary bonus offered by the airport was “no substitute for an hourly pay increase”.