Mobile network Three has acknowledged it is experiencing “technical difficulties with voice, text and data”, leaving many customers offline.
The problems appear to have started on Wednesday evening, according to the Down Detector website.
Customers across the UK have taken to social media to complain about the loss of service.
Three apologised for the problem and said it was “sorting this out right now”.
The problems, which are nationwide, started after some maintenance work on Three’s network infrastructure.
It is not sure how many of its 10 million customers are affected.
So many customers tried to access the status checker on Three’s website that it was temporarily unavailable on Thursday morning.
A queuing system has been switched on, to limit access to the tool.
“History shows that once service is restored people quickly forget about the issues,” said Ben Wood, an analyst at the CCS Insight consultancy.
“The challenge for Three UK will be getting its network back online reliably. Often it can take time for things to stabilise after such a massive outage, which can lead to intermittent service for a period of time after the original problems.”