Customers with O2 are experiencing difficulties accessing voice and data services after the firm admitted there are issues with its network.
On a status page for the network O2 said staff were working to fix the problem, which was logged as starting at 1:30pm, but no details on the scale of the outage or the cause have been provided.
"We are currently seeing a problem on our network affecting some of our customers. Those customers affected will have difficulty making or receiving calls, sending texts or using data," it said on the service page.
"Our engineers are dealing with the problem as a priority and we hope to restore full service as soon as possible."
The firm also confirmed that the issue was "not geographical" suggesting it is not an issue with its masts but is affecting random users in different locations.
The incident is just one of a number of network outages that have blighted UK mobile firms in recent years.
O2 itself suffered a major network outage that left thousands of customers across London, Kent and Sussex without access to 2G and 3G services, after theft and vandalism at one of its sites in east London in 2011.
Vodafone also suffered a significant network outage after a technical facility was broken in to and vital equipment stolen in early 2011.
Meanwhile earlier this year Orange had to apologise to customers for a network glitch that left scores of users without access to key data services including 3G access.
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