Millions of smartphone users in the UK have lost their data services after the O2 network suffered technical problems.
O2 has 25 million customers, but also provides services for the Sky, Tesco, Giffgaff and Lycamobile networks which have another seven million users.
Many users took to Twitter to complain about the difficulties, which were first reported at about 05:30.
An O2 spokesperson said a software issue identified by a third-party supplier was to blame for the outage.
Ericsson, the Swedish firm that has close links with O2’s Spanish owner Telefonica, told the BBC it was aware of an issue and it was investigating.
O2 added: “We believe other mobile operators around the world are also affected. Our technical teams are working with their teams to ensure this is fixed as quickly as possible.
“We’d encourage our customers to use wi-fi wherever they can and we apologise for the inconvenience caused.”
O2 has the UK’s second-largest mobile network after EE, which is now owned by BT.
The company has said voice calls are not affected by the problem, but some O2 customers say they cannot make calls or send texts either.
The outage is having knock-on effects for other services that use the O2 network, including Transport for London’s electronic timetable service at bus stops, which has stopped working.
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