Translink has launched an investigation after one of its Ulsterbus drivers apparently refused to let a blind man on his vehicle because he was accompanied by his guide dog.
A passenger on the service from Lisburn to Belfast said a man, aged in his late 50s or 60s with a white cane and what looked like a guide dog attempted to board the bus at Lisburn train station.
"The driver immediately told him that dogs were not allowed on the bus and he couldn't get on," said the man.
"He attempted to explain that the animal was a guide dog, but the driver just said no dogs were allowed and the bus left without him."
He added: "The dog looked to me to be a guide dog with the proper harness and a yellow bib, and I was shocked that someone would be treated in the way that man was."
Ian Baxter Crawford, spokesman for Guide Dogs NI, said: "While this may be surprising to most, it is not surprising to me.
"This kind of thing goes on, but it is surprising to hear that it is Translink that is involved."
Ian said his organisation, along with others, worked with Northern Ireland's primary public transport provider on training on disability awareness and on improving accessibility in its buildings and on its vehicles.
He continued: "We have a close working relationship with Translink dating back at least 10 years. Our training provides advice and assistance on how people using canes or with guide dogs should be treated.
"There can be and there still is confusion on the differences between people with guide dogs and assistance dogs, but the law is the same for both in that they should be allowed onto any public transport." Mr Crawford said Translink had gone "above and beyond" to cater for people with disabilities and to inform its staff.
He added: "We have been working with them on improving services; for example we are trying to get audio and visual announcements for stops on buses and they are supportive of that. But it is a large organisation and many people do work for it."
Joe Kenny, from the Royal National Institute for the Blind, added: "Without knowing the full details of the story, if a guide dog was wearing its proper harness it should be allowed onto public transport.
"We work closely with Translink and others to raise awareness of the difficulties faced by blind and partially sighted people in Northern Ireland."
Translink has spent millions upgrading its facilities and improving its vehicles to make them accessible to as many people as possible.
On the bus operator's access guide it guarantees to welcome guide and assistance dogs and their owners onto all services.
A Translink spokeswoman said an investigation was underway and CCTV footage was being examined.
She said the driver of the vehicle believed that the man did not want to get on the bus.
A statement added: "An investigation into the circumstances of the incident is being carried out by the service delivery manager in Lisburn.
"We would like to thank the passenger concerned for bringing our attention to this."