The London Ambulance Service says it expects its 999-call computer system to be back up and running "in the next hour or two."
A spokesman said that the technical issue had been identified and that they were now bringing the system live again.
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The London Mayor has made some forthright comments about Britons who have been lured to join Islamic State.
Bright spells with rain, sleet and snow showers at the weekend.
As soon as the information went missing in early January, the Ministry of Justice say they launched an urgent investigation.