London Assembly member Darren Johnson has obtained data on modelled fire response times before and after the proposed cuts come into force.
The following wards will see first response times worsen by over a minute.
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The problems appear to have been caused by a power supply problem in Queen's Park in north west London, which has affected signalling.
Cloudy conditions continuing overnight, with some patchy outbreaks of rain.
The mayor has revised his proposals for a six-mile East-West cycling superhighway in a bid to cut traffic delays.