Arriva said this afternoon that 99% of its buses are equipped with air conditioning units.
"The company is also undergoing a process to install additional units on all buses, which work directly on the drivers cabin and most of Arriva’s fleet has already been fitted with such units. This process will be complete in the coming weeks."
The company was reacting after the GWU directed drivers not to drive buses which do not have a functioning airconditioning unit.
Arriva apologised to passengers who may have been affected by the action ordered by the union.
"Arriva officials will be meeting union representatives this afternoon to find the best way to mediate the current situation until the formal talks planned for the middle of this week.
Meanwhile, the GWU said this morning that its action is indefinite. It also said that the company had tried to intimidate the drivers, with members of the management threatening them with disciplinary action.
The union and the company are currently in talks on the drivers' working conditions.
A passenger at the Marsa Park and Ride said she could notice a number of delays this morning.
"The buses usually leave from three village and are quite frequent. This morning just three buses passed in an hour, and they were all full up because a number of buses did not operate the route.
She said drivers had told her that their colleagues were not driving buses without airconditioning because a number of passengers had been taken ill as the temperatures soared and airconditioning was not used.
"The people then take it out on us" one driver exclaimed.
The extent of the industrial action is not clear. The Arriva spokesman could not immediately comment.
The action comes a week before Arriva marks a year since it launched its service in Malta. The company is still using a number of newly-imported buses or buses from the old service which are white and have not had airconditioning installed yet. Passengers, however, have also complained about airconditioning not working on other buses.