TRANSLINK'S internal computer systems have been experiencing problems following a suspected 'ransomware' cyber attack.
The public transport firm said it is working to resolve the issues and there is "no evidence that any data has been compromised".
The incident has been reported to police. The National Cyber Security Centre, part of GCHQ, is also examining the hack.
Investigations are continuing into the cause of the problems, but a source told The Irish News that a ransomware virus was suspected.
Ransomware is a computer virus which attempts to block data stored on an infected machine and demands payment before allowing the user to regain access to their information.
The source claimed Translink's IT network had been in "lockdown" since Wednesday afternoon, with some staff unable to log into computers.
In a statement a spokeswoman for Translink said: "We are experiencing difficulties with our internal IT systems.
"We are working to resolve these issues as quickly as possible.
"Investigations are ongoing 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."
A spokesman for the National Cyber Security Centre 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."
A police spokeswoman said: "PSNI have received a report of a suspected cyber related incident and are working with Translink to resolve it."
It is not the first time public bodies in Northern Ireland have been targeted in cyber attacks.
In 2016, figures obtained by The Irish News showed how Queen's University Belfast had suffered three ransomware attacks within a year.
On one occasion it paid a £400 ransom after hackers targeted a PC running Windows XP and encrypted documents and images.
Ulster University had identified 22 ransomware attacks since June 2015, while several councils also revealed that they had been targeted by ransomware.
