O2, Britain's second biggest network, and Japan's Softbank reported network outages on Thursday, leaving many customers unable to access internet services.
Companies that share O2's network - such as GiffGaff, Sky Mobile and Tesco Mobile - are also being hit by the problems.
The problems seem to have started at around 8:00 AM this morning and are ongoing at the time this article was posted.
The outage tool - which allows customers to check if their area is affected - was also down.
EE U.K. mobile network EE has confirmed that 16 U.K. cities will be able to enjoy blisteringly fast 5G data speeds by the end of 2019.
Borje Ekholm said: "The faulty software that has caused these issues is being decommissioned and we apologise not only to our customers but also to their (other network's) customers".
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The data outage has meant O2 has seen a strain on users making and receiving voice calls too.
In a statement, O2 said: "All O2 technical teams are working closely with one of our third party suppliers who has identified a global software issue in their system which has impacted data services".
Thousands of O2 customers in Northern Ireland have been left without data on Thursday morning due to a technical issue.
Millions of people have been having problems with their mobile phones this morning, after the O2 network went down.
However, when the network was down for seven hours in 2015, the company did not offer compensation to all customers, instead saying payouts would be assessed on a "case-by-case" basis. "We fully appreciate it's been a poor experience and we are really sorry". However, some customers are reporting voice issues as well.
Earlier, O2 said it suspended planned work or updates to its systems and network. "Our technical teams are working extremely hard to resolve this", the company said.
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