A section of Translink workers have voted in favour of industrial action amid an ongoing pay dispute with management.
Of members of the Transport and Salaried Staffs' Association (TSSA) who took part in the union ballot, 60% backed strike action with 80% supporting action short of strike.
The union said it would hold further discussions with members before deciding what option to take.
The row involves employees at management, professional and technical (MPT) grades.
Translink has described the dispute as "regrettable" but has stressed it only involves a small number of its overall workforce.
TSSA claims a pay increase offer made to MPT employees was 1% lower than other grades within the company.
TSSA general secretary Manuel Cortes said the "unprecedented ballot" demonstrated the "frustration, bewilderment and disillusion" among its members.
"All our MPT members want is to be shown the same respect as their colleagues with regards to their pay," he said.
"I hope this strong mandate for industrial action will now concentrate the company's mind on finding a resolution.
"TSSA reps will seek further discussions with our members on what action we will take.
"In the meantime, we remain available for talks under the auspices of the Labour Relations Agency. We hope Translink will talk to us there."