United Continental Holdings' employees represented by the International Association of Machinists (IAM) have rejected a tentative agreement.
The joint agreement covered more than 28,000 ramp and stores, and public contact employees at its United Airlines, Continental Airlines and Continental Micronesia subsidiaries.
United and the IAM will seek US National Mediation Board mediated talks following the rejection.
"We believe these tentative agreements are in the best interests of our co-workers and the company," says United.
The airline and union reached the tentative agreement in February.