Calls and texts were, initially at least, unaffected but some users later reported they were unable to place telephone calls.
"We are aware our customers are unable to use data this morning", the company said on its official Twitter account.
O2 says it is trying to resolve the issue as fast as possible.
"I want to let our customers know how sorry I am for the impact our network data issue has had on them, and reassure them that our teams, together with Ericsson, are doing everything we can", Evans said in the written apology.
Tens of millions of smartphone users in Britain lost internet access today after a software glitch brought down O2's data network. Adding MVNO customers to O2's 25 million direct customers, up to 32 million people are affected.
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Companies that share O2's network - such as GiffGaff, Sky Mobile and Tesco Mobile - are also being hit by the problems.
Thousands of O2 customers have not been able to get online on Thursday morning following a technical fault with the network.
"During December 6, 2018, Ericsson has identified an issue in certain nodes in the core network resulting in network disturbances for a limited number of customers in multiple countries using two specific software versions of the SGSN-MME (Serving GPRS Support Node - Mobility Management Entity)", the supplier said.
The company confirmed the network issues on their website, saying: "Our technical teams are investigating reports of issues when using data. Ericsson sincerely apologises to customers for the inconvenience caused".
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. Work is now ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.