Cardiff Bus put people at risk for several weeks by failing to use coronavirus-killing disinfectants, a drivers' union has claimed.
The council-owned firm switched to a product proven to kill coronavirus on 9 April, having previously used another it believed was suitable.
Unite claims that it had not followed guidelines for a month.
Cardiff Bus said the health risk was "limited" and it would continue to review its cleaning processes.
The UK government published guidelines in March which recommend organisations in non-medical settings use disinfectant that is "effective against enveloped viruses".
Unite's regional organiser Alan McCarthy said a Cardiff Bus employee raised concerns about the disinfectant being used on buses in the city last week.
He said Cardiff Bus, which brought in enhanced bus-cleaning measures on 11 March, told staff on Tuesday it had switched to an "enhanced" disinfectant.
"It is crucial that Cardiff Bus now fully reassures the general public that its cleaning regime is of the required standard," Mr McCarthy said.
"It also needs to fully address the internal failings that have clearly played out here."
Cardiff Bus said in a statement it had reviewed its cleaning process after the outbreak of coronavirus.
"The decision was made at that time to continue with the use of the current sanitizer-disinfectant, but at the same time, increase the frequency of cleaning.
"It was believed at that time, that the product was suitable and contained ingredients that were active against enveloped viruses."
Cardiff Bus said it continued to use the product "until it was highlighted that there was a more effective coronavirus-killing product available on the market.
"As soon as the managing director was made aware that there was a confirmed coronavirus-killing disinfectant available, it was immediately sourced, replacing the current product and all operational buses were deep cleaned," they added.
Cardiff Bus said the council's environmental health department had confirmed the overall health risk was "limited" for the weeks prior to 9 April.
This, it said, was due to the increased physical cleaning, drivers following good hygiene practices and the limited number of passengers who had travelling on the buses.
Additional personal protective equipment which had been made available to drivers would also have reduced any risk.
"The company will continue to review and improve cleaning processes in line with government guidelines," the firm said.
Jean-Yves Maillard, professor of pharmaceutical microbiology at Cardiff University, also believes that the risk to the public would be low, explaining that an "enveloped virus" like the coronavirus has an outer shell that is "extremely fragile".
However, Unite said it was not acceptable that such a large bus company had not been complying with the guidelines on anti-viral disinfectant for almost a month.
It said it had been told by its members that other major bus companies in Wales had been complying "for some time".
First Cymru said it has been using anti-viral disinfectants since before the start of the pandemic, with Stagecoach South Wales saying it introduced anti-viral disinfectant on 5 March.
Other public transportation companies, including Transport for Wales, also confirmed the use of anti-viral disinfectants since before, or from the start of, the pandemic.
A spokesman for Cardiff council said that, while it was the sole shareholder in the company, the authority was "not involved in the day-to-day running of Cardiff Bus".
The council said it had recently shared Public Health Wales advice on the recommended cleaning products when contacted by Cardiff Bus, having already been advised that an enhanced cleansing regime was in place at the company.
A Welsh Government spokesperson said: "We expect all organisations and individuals (including local authorities) to follow government and Public Health Wales advice in relation to Covid-19."