SIPTU says it will not re-enter talks with Bus Éireann until all pre-conditions are removed.
The Transport Minister Shane Ross has asked management at the company and trade unions to engage in talks "with a blank sheet of paper" - but has insisted he can't and won't directly intervene.
The union says conditions laid down to staff in a letter from the company are not fair and don't form a good basis for further discussions.
SIPTU Sector Organiser Willie Noone says Minister Ross isn't clear on the matters involved in the dispute: "It appears the Minister for Transport is speaking out of both sides of his mouth at the moment.
"Because on one hand he has said he can't understand why the unions are not talking and he is also saying that he is not going to get involved in the talks and he is not going to be involved in any industrial relations matter.
"That's totally disingenuous."