Translink bus drivers have said they may go on strike unless the company withdraws its new policy on attendance.
Strike action could effect services around Christmas
The Transport and General Workers' Union said Translink introduced the new scheme without full consultation.
Translink said the policy was introduced after consultation and discussions were still ongoing.
If strike action is taken, it could affect services around Christmas time. The union said it intended balloting members if there was no resolution.
Sean Smyth of the Transport and General Workers' Union said: "If you are absent more than once in three months, you can be brought into a disciplinary situation.
"There is nothing there to help the drivers.
"When they do come back from the sick, they have to go for a formal interview with their manager.
"They are not allowed to be accompanied by a colleague. It is new - we have never done this before.
"At least let us have a colleague with us for the first couple of occasions so that we can get used to this new policy."
The unions are giving Translink seven days to withdraw the policy and enter into negotiations with them, or they will ballot members.
A Translink spokesman said: "Translink's 'Attendance Management and Rehabilitation Policy', effective from 1 September 2006, was introduced after consultation.
"Discussions on the implementation of the policy have been ongoing.
"We have invited the trade unions to join us for further discussions in relation to this policy."