Angry customers have taken to social media to vent their frustration at problems with Translink's app.
The mLink app allows passengers to use pre-paid tickets on public transport.
However, some customers experienced difficulties on Monday and Tuesday.
Hey @Translink_NI any idea when your Android App will start working again? Spent another 30 mins this morning trying to buy my weekly ticket...— Garth Gilmour (@GarthGilmour) September 4, 2018
Translink say the problem was a result of "supplier capacity issues" which affected operators in the UK and parts of Europe, adding that the app is now working again.
In a statement, Translink acknowledged the purchase of new tickets "was not possible in some cases".
They have urged customers who are experiencing problems to contact them.
#mLink is not working on iOS this morning again but absolutely fine on Android. My Wife usually buys a monthly ticket and has now been unable to the last two days. When will this be rectified @Corethree ? #GliderBelfast— Jonathan Godfrey (@GodfreyJonathan) September 4, 2018
@Translink_NI @TranslinkMetro hate to bring it up but not only has your app crashed BUT your busses are still running late was told by your “live” app that the bus would be 10mins early it is now 10mins late! Would love an explanation? And it’s probably not that there’s traffic?— Chris Grant (@ludachriss) September 4, 2018
Yeah I figured as much! Conveniently they can only really to you about it after 9am when most people will have had to buy one to get to work already.— ClaireW (@clairedotw) September 3, 2018