Dublin Bus and Exterion Media have come under fire for displaying ads on buses from EU MEP candidate Gemma O'Doherty.
The ads claim "It's time to take Ireland back" on behalf of Anti-Corruption Ireland and have been spotted around the city on the side of buses.
Exterion Media, who manage advertising for Dublin Bus, have called the ads "acceptable" under its advertising criteria.
Political advertisements do not come under the ASAI guidelines.
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But many have taken to social media to blast both Dublin Bus and Exterion Media for displaying the ads of a political candidate who has previously spoken out against Islam and Muslims.
NBRU General Secretary Dermot O'Leary has written to the CEO of Dublin Bus and said the union fully support any driver who refuses to drive the buses with the ads displayed.
He said: "I have written to CEO of Dublin Bus (below) outlining our views on decision to carry these ads.
"NBRU has been representing Bus Drivers for generations, we will of course fully support any driver that has a conscientious objection to being forced to drive these Buses."
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He shared the letter, which states that the advertisements are a "direct contradiction" between the views of the political candidate and Dublin Bus stance as a multi-cultural employer.
It said: "The NBRU utterly condemns racism in all its formats, we hold no truck with those that use the colour of one's skin, or their country of origin as a basis to say they don't belong here."
In a statement, Exterion Media said: "Exterion Media manage and operate the advertising formats owned by CIE incl. CIE's Group Companies (Bus Eireann, Dublin Bus and Irish Rail).
"These advertising formats are available to all brands, businesses and organisations, including those of a political nature.
"In relation to political and election campaign advertisements, provided a political party, organisation or election candidate is not precluded or restricted by law or other prohibition from advertising or promoting, we remain wholly impartial on the political position or view of that party, organisation or candidate.
"We consider the advertising copy for anti-corruption Ireland/Gemma O'Doherty to be acceptable under these assessment criteria and we would not reject such copy solely on the grounds of it being of a political nature or from a specific candidate.
"In this current election period, there are/have been eight separate campaigns running on CIE advertising formats for both political party and independent candidates."
