Dublin Bus passengers could face major delays as drivers will refuse to use the Port Tunnel next week.
They have reacted angrily after being banned from listening to the radio while driving as part of a new "zero tolerance" policy.
The move - to start next Tuesday - will affect routes, including the 747, 41x, 33x and 142, and could lead to major traffic congestion on routes to the airport.
The tunnel ban will take place on the same day as the next Luas strike.
The National Bus and Railworkers Union has written to Dublin Bus to inform the company that it has advised its members not to use the tunnel for health and safety reasons.
It said drivers had used their personal radios to deal with emergency situations, but they would not use the tunnel as long as the "ridiculous and illogical" radio ban stays in place.
"Your prohibition on FM transistor radios means that in an emergency situation our members would be unable to tune into FM radio stations to hear safety instruction, in case of incident," said the union in a letter to Dublin Bus.
"We cannot expose the travelling public and our members to this level of risk, and we must adopt the utmost caution when it comes to the safety and welfare of bus drivers and customers alike."
Drivers have already refused to cooperate with central control after the ban began yesterday.
The new policy prevents them from using any electronic devices, including radios and mobile phones, while driving.
It is understood that up to 90pc of drivers use radios, but sources said even greater numbers brought them in to work yesterday "as an act of defiance".
Their refusal to connect with the control room could weaken management's control of the service.
For instance, drivers could not be instructed to take alternative routes if there was an accident ahead.
The control centre would also be unaware if drivers took their breaks late, or had fallen ill at short notice.
In addition, it could not instruct drivers to travel straight to a terminus without picking up passengers to make up for time lost from a delay.
General Secretary of the NBRU Dermot O'Leary described the policy as "stupidity of the highest order".
"The radios break up the monotony for drivers," he said.
"Every bus in the world has a radio on it - including the Aircoaches - bar Dublin Bus."
"It's going to cause problems as the week goes on," he said.
He said the union could not argue against national legislation around the use of mobile phones while driving but "totally and utterly refuses" to accept the inclusion of radios on the list.
Dublin Bus said it had a long-standing policy prohibiting the use of mobile phones and electronic devices while driving.
"This is part of driver safety training," said a spokeswoman.