Stansted faces a threatened walkout by baggage screening staff over the August bank holiday weekend.
The GMB union said workers were unhappy with a pay deal offered by Mitie, a company which conducts security checks on hold luggage.
Staff are being offered a 2% pay rise, but will lose their overtime rates, the union told the BBC.
The result of the vote is expected to be known on August 12.
The GMB said 39 of outsourcing company Mitie's 41 workers at the airport were in the union.
Mitie has yet to comment on the ballot.
Union organiser Gary Pearce said: "The GMB expects a vote for action and, if this is the case, the first action will likely take place over the August Bank Holiday weekend."
Workers do an average of six hours overtime a week in the summer, he said.
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