Customers have complained that mobile phone network EE has been down for most of the day across many parts of the UK.
DownDetector, a website which records network outages, shows that Sheffield, Bradford and Manchester are some of the worst affected areas.
Customers have said that they are having trouble making and receiving calls.
An EE spokesperson has said that emergency calls, as well as all data and messaging services, are 'working as normal' and apologised for any inconvenience caused.
The company said they are working to the problem 'as quickly as possible'.