A series of one-day strikes have already been stagaed by members of the Unite union at First Aberdeen.
The dispute is linked to changes to pay and conditions.
Drivers have now agreed in principle to hold up to four weeks’ continuous strike action starting early next month.
A week-long stoppage is already set to start next week.
First Bus said in a statement it was “extremely disappointed” by the decision.
The strike deal was reached following a mass meeting of drivers.
Unite has claimed safety will be compromised as drivers will be asked to work longer shifts.
First Aberdeen managing director Andrew Jarvis said: “We await formal notification of this from Unite to detail the exact dates of any further action, but focus now will turn to planning to deliver services on our network as much as we possibly can to keep our customers moving and limit the disruption.
“We are disappointed that despite significant improvements to the original offer being proposed to Unite and their members at the end of the previous set of talks, this was still not enough to bring an end to the dispute.”