Some 85 west Belfast black taxi drivers have lost their jobs since Glider buses were introduced, a spokesman has said.
Taxis have lost trade as some Glider passengers are not buying tickets and regard it as a free service, said West Belfast Black Taxis.
Translink said it had no significant concerns about fare evasion on Gliders.
This was verified, it added, by ticket inspectors who carried out spot checks "at different times and locations throughout the day".
Second-hand Hackney-style black taxis have been providing a transport service in west Belfast for more than 50 years.
But West Belfast Black Taxis said it now had 130 drivers on its books, compared to 215 when the Glider service began in September 2018.
"Our main criticism of the Glider would be that due to fares not being taken on board the service," said Brian Barr, transport manager for West Belfast Black Taxis.
"A lot of people feel: 'I'll take a chance. I'll get on it free.'
"If they see a ticket inspector they will then get off."
The issue could spell the end for his company, added Mr Barr.
"If things persist the way they are going, eventually the Belfast black taxis service could ultimately fade away," he said.
"That is our worry. It's a real big concern at the moment."
U-turn on concessionary deal?
Mr Barr accused the Department for Infrastructure (DfI) of going back on a deal that would have allowed black taxis to provide feeder services for passengers from outlying housing estates in West Belfast to the main Glider route.
"They assured us that we would be providing the feeder services, from Whiterock, Glen, and Shaws Road," he said.
"We should have been used. We are part of the public transport network.
"We have the same licences as Translink. So I don't understand [why] at the last minute, there was a U-turn done and we were not advised."
The Department for Infrastructure confirmed that the talks had taken place.
"The Belfast Rapid Transit team engaged with West Belfast Black Taxi representatives in relation to them having a role in feeder services," said a department spokesman.
"However these discussions were not successful and this option was not considered further in the design of the system."
While taxi firms were part of a community enterprise, they were victims of unfair competition as Translink received a huge subsidy from the Stormont executive, added Mr Barr.
The Glider system was funded by £94.4m funded by the Department for Infrastructure, which included:
- £48.7m infrastructure improvements
- £24.2m on buses
- £7.4m on new ticketing arrangements
Black taxi operators also feel disadvantaged because they are not included in Translink's concessionary fare scheme providing refunds to carriers for passengers over the age of 60 who can travel for free.
The DfI was working with the Belfast Taxis Community Interest Company to see whether it meets the audit requirements of the NI Concessionary Fares Scheme, said a department spokesman.
"This work is ongoing," he added.
Translink said the Glider service had resulted in a 30% increase in journeys along its network, representing 45,000 additional journeys a week and the removal of 1.67m car journeys, which both reduced congestion and improved city air quality.