Translink's system are reported to have been "crippled" by a virus placed inside the company's intranet by hackers.
The "very serious" cyber attack was first noticed on Wednesday and it is understood those behind it are 'holding the travel firm to ransom'.
Two separate sources told Belfast Live it has rendered Translink's internal computer systems unusable.
A Translink spokesperson said bus and train services have not been affected and that they are working to try and resolve the issue as soon as they can.
They added: "We are experiencing difficulties with our internal IT systems. We are working to resolve these issues as quickly as possible.
"Investigations are on-going and there is no evidence that any data has been compromised.
"Bus and train services are not affected.
"Passenger and timetable information is available on the Translink website, Journey Planner and through our Contact Centre."
The UK's National Cyber Security Centre is working on the issue.
A spokesperson said: "We are aware of the cyber incident affecting Translink and are working with partners to get a full understanding of the situation.
"The company have said that transportation has not been affected and that they are still able to take payments."
The PSNI and Department of Infrastructure have been contacted for comment.
