Private taxis will be allowed to use bus lanes as part of a 12-month trial in Belfast.
Some taxis are already permitted to drive in the lanes. Most will now have access - including to the lanes earmarked for the new Rapid Transit System.
Private taxi firms have been lobbying for a change.
The Department for Infrastructure has said it is aware of opposition and will continue to engage with people on the issue.
Michael Dornan from the union Unite said: “Over the years, the Metro service has built up a big population of transport users taking people out of cars and into public transport because it would have eased the congestion and freed up the roads for more commercial users.
“This is all going to be destroyed.”
Meanwhile taxi company owner William McCausland said: “It’s going to be a great benefit for our drivers and our customers.
“The response times to the customer for us to be able to get to them, especially around the city centre with the 12-hour bus lanes coming in, is going to be reduced.”