Users of O2 services, who were unable to access the internet or apps that require an internet connection, found their way on to social media to complain about the outage, which also knocked out London bus live information displays.
The outage kicked off around 4.45am this morning, with services that rely on O2's network also suffering as a outcome.
The outage is also causing other disruptions, such as issues with public transportation.
'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".
O2 has 32 million customers, including people signed up to Tesco Mobile, Sky, Lycamobile and GiffGaff.
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One of the largest United Kingdom phone networks, O2, is investigating a network outage that has rendered mobile data inaccessible to the company's 25 million customers.
Marielle Lindgren, CEO Ericsson UK & Ireland said: "The cause of today's network issue is in certain nodes in the core network resulting in network disturbances for a limited number of customers across the world, including in the UK".
O2's 4G cellular network has been restored, the British mobile operator said on Friday, following a software glitch which caused smartphone users to lose internet access.
SoftBank said the outages stemmed from problems at exchanges for its high-speed wireless LTE network, while O2 blamed an issue in a third-party supplier's system that was impacting services globally. "We fully appreciate it's been a poor experience and we are really sorry".
O2 chief executive Mark Evans said his teams were working hard with Ericsson to find a swift solution. The company also apologized for the inconvenience.