London Ambulance Service was hit by a 'computer system crash' on New Year's Eve, ITV News London understands.
Officials confirmed there was a systems fault in the early hours, though staff are trained for such situations, and they continued to prioritise responses as normal.
Calls were reportedly logged manually between 12.30am GMT and 5:15am.
Ambulances had to be tracked manually using maps, and operate without satellite navigation systems, according to reports.
It is believed that Command Point, the service's call logging and tracking software, crashed.
LAS said its systems were back online and it is investigating the incident.