Around 32 million people are seeing lost in data services after the O2 network suffered a disruption to service.
'A statement reads: "The network issue we are now experiencing remains a top priority for us and we are working as quickly as possible to fix the issues".
The technical problem left millions of workers without access to emails and news on their morning commute and enraged those who wanted to access social media.
O2 is the second largest mobile network in the United Kingdom, and supplies connectivity for other smaller networks including Sky, Tesco Mobile, Lycamobile, and GiffGaff.
"As a result of this incident, we are experiencing a high demand on our network which means some customers may have issues making and receiving voice calls".
OUTAGE UPDATE 10.45am: One of O2's third party suppliers has identified a software issue in their system. In Japan, the firm suggests 3G networks kicked in to partly compensate for the 4G issues, though the same could not be said for the UK.
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It has confirmed engineers are working to fix the issue.
Telefonica, the Spanish owner of O2, said more than 12 hours after the outage was noticed in London at around 5.00am that it had been assured by Ericsson that "full service will be restored for customers by (Friday) morning".
"We'd encourage you to use Wi-Fi wherever you can and we are really sorry".
"We're aware that our customers are unable to use data this morning", the company admitted in a statement. O2 says technical teams are working "extremely hard to resolve this".
Depending on the circumstances, it may be appropriate for your provider to offer you some money back while repairs are being carried out.
In cases where you have been without service for some time, you may also have the right to leave the contract without penalty.