Millions of O2 customers have been unable to to make or receive calls or data since around Midday today.
The mobile phone company says it has now fixed the fault but that customers may still experience problems as full service returns.
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Diane Abbott dismissed the idea of the Labour leader standing down even if the party loses both seats in the upcoming by-elections.
Levels of safety are not high enough in prisons, meaning that reform of inmates cannot take place, Prisons Minister Sam Gyimah has said.
The Government's Prevent campaign is "simply not trusted" by many British Muslims, the Independent Reviewer of Terrorism Legislation said.