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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London mayor Sadiq Khan and Prime Minister David Cameron both gave speeches in Roehampton at the arrival of Britain Stronger In Europe bus.
Millwall fans who forced Barnsley supporters to flee as they tried to break into the opposing team's end at Wembley could face life bans.
Some sunny spells are likely later in the west following a dull start.