Mobile network Three is investigating an issue with its network which has left many users unable to make calls or send messages.
Some customers began reporting issues with the network on Wednesday night.
The company apologised and said it is investigating the incident.
“Three is currently experiencing technical difficulties with our service across voice, text and data, which means that some customers will be experiencing an intermittent service,” the company said in a statement.
“Our engineers are working on the issue now to fix the problem as soon as possible. We are sorry for the inconvenience caused to our customers.”
As well as its mobile network, Three’s website was also unavailable on Thursday morning, instead displaying a message which said it was down for “essential maintenance”.
According to online service monitoring website DownDetector, the issues with Three began at around 11pm on Wednesday.
The majority of users who flagged the issue with DownDetector cited problems with mobile internet.
The incident came on the day another mobile operator – O2 – became the latest to begin the rollout of its next-generation 5G network.