Thousands of O2 mobile customers have been without service for hours after a crash in the firm's network.
Customers across the country have been enduring network loss since Wednesday lunchtime, leading the company to apologise to its service users.
But the firm was unable to identify the exact cause of the outage, which has affected some - but not all - O2 customers.
In the latest update, an O2 spokeswoman said: "We can confirm that the problem with our mobile service is due to a fault with one of our network systems, which has meant some mobile phone numbers are not registering correctly on our network.
"As a result, some customers are having difficulty making or receiving calls, sending texts or using data.
"We, and our central supplier, have deployed all possible resources and are working through the night to restore service as soon as possible.
"We apologise again to customers affected and will provide further updates as soon as we can."
GiffGaff and Tesco Mobile customers, who rely on O2's masts, have also been affected.
Sky News business correspondent Tadhg Enright said the situation was "immensely embarrassing" for the company.
"If this continues well into the next working day, this will become a far bigger reputational problem for them," he added.
The O2 spokeswoman said the problem was not restricted to one region: "The problem is an issue within part of our core network that is preventing some mobile phones from successfully connecting.
"The problem is not location-specific. All possible resources across our and our suppliers' engineering teams are being deployed to restore service as soon as possible."
Customers have taken to Twitter over the outage.
One Twitter user, Kelly Jones (@kelly_92), tweeted: "Having a phone that hardly works usually is annoying, but this whole no signal on o2 all afternoon is beyond irritating."
Another, Bethany Kemp (@Krazy_Kempy), said: "It's ridiculous, haven't had a signal since 1pm. @O2 you are going to have to do some apologising if you don't want to lose customers."
While some saw the humorous side, @Gazbelfast tweeting: "What did one O2 customer say to the other? Nothing."
O2 have been keeping customers updated through their service status page.
O2 have around 23 million customers in the UK.
It is unclear exactly how many have been affected by the problems but a spokeswoman said she estimated it was "thousands" at least.