O2 has said a "full service" will be restored by Friday morning following a data outage which affected thousands of users across South Yorkshire.
The problem started early on Thursday morning and has still not been fixed, with O2 customers unable to use 3G and 4G services to access the internet.
In a joint statement from the bosses of O2 and Ericsson, a provider for the phone operator, they apologised to customers for the "poor experience" they faced and said the network should be back up and running tomorrow morning.
Mark Evans, the chief executive officer of Telefonica (O2), said: "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.
"We will continue to work with Ericsson, through the night, who have assured us that a full service will be restored for customers by the morning. We fully appreciate it's been a poor experience and we are really sorry."
Ericsson boss Marielle Lindgren added: “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.
"We have been working hard on resolving the UK data issue since early this morning.
"The faulty software that has caused these issues is being decommissioned. Our priority is to restore full data services on the network by tomorrow morning. Ericsson sincerely apologises to customers for the inconvenience caused."
The data outage has meant O2 has seen a strain on users making and receiving voice calls.
The problem has also hit customers of Sky Mobile, Tesco Mobile, Lyca Mobile and GiffGaff - all of which use O2's infrastructure.