BUS Eireann cannot guarantee a full service from Sunday as it does not know what type of industrial action it faces.
A spokesman said it was unable to reassure customers as it cannot draw up a concrete contingency plan because it is unclear what sort of staff cover it will have.
The National Bus and Rail Union has warned it will take industrial action if the company goes ahead with plans to cut premium rates and overtime from Sunday.
But it has not said what form the action will take, which could mean anything from a one-hour stoppage to an indefinite strike.
However, even if members down tools, the other main union, SIPTU, is not due to mount pickets until Thursday week so there should be a reasonable level of service until then.
From next Thursday, there will be little or no service if members of both unions mount an all-out strike as sources said the company is unlikely to hire in drivers.
There were indications of a last-minute intervention by the Labour Relations Commission in a bid to avert a strike.
SIPTU has warned that the threatened strike could spread to Dublin Bus and Irish Rail if management forges ahead with its plan to slash €9m from its payroll costs.
It said the entire CIE Group could be affected after its 1,000 members at the bus company backed industrial action by an overwhelming majority yesterday.
The SIPTU members voted by 91pc in favour of a strike over company plans to impose cuts in premium pay and change work practices.
The union said its members voted for strike action rather than "less effective industrial action due to the seriousness of the issues at stake".
"This decision is a direct response to the threat by management to dictate change and cut the pay of our members," said organiser Willie Noone, following the ballot result.
He accused the company of trying to fast-track the changes through the Labour Court without going through the usual talks process.
However, Bus Eireann claims the unions have refused to enter talks at the Labour Relations Commission about key savings to overtime, shift rates and premium payments.
A strike is not expected to affect school bus services for 114,000 children. Traditionally, school bus drivers have been excluded from industrial action and are allowed to pass pickets to ensure children's education is not disrupted.