Thousands of British Airways cabin crew started voting today on whether to launch a pre-Christmas strike in protest at "far-reaching" changes to working practices.
New employment contracts came into force today despite opposition from Unite, which warned that they were "unworkable".
More than 12,000 ballot papers were issued and the result will be announced on 14 December, raising the threat of strikes just days before Christmas.
Unite said it was concerned that the changes would increase stress levels among cabin crew because fewer staff would be on duty during flights.
The union said services to passengers would suffer and the health and safety of workers would be hit.
National officer Brian Boyd said: "The way to deliver change is through negotiation, not imposition. Cabin crew are angry - they care passionately about this business, but feel they are not being listened to on serious matters of service delivery.
"This business is all about its customers and the crew do not want them to suffer as a result of these changes."
Unite called on BA to abandon the new contracts and work with the union to try to achieve a negotiated settlement.
National officer Steve Turner said: "We have lodged a formal letter with BA on behalf of our 12,000-plus cabin crew members who strongly oppose the company's plans to force through cuts.
"Crews being forced to work under protest retain their legal rights to pursue a reversal of the imposition as well as significant compensation, and we continue to fight this case in the High Court."
The union announced it had instructed a team of employment specialists to monitor the impact of the new contracts in the coming weeks.
BA is reducing jobs and freezing pay under cost-cutting plans in the face of huge losses and a downturn in the aviation industry.
The company maintains the changes are vital for its future.