Dozens of firefighters are tackling a blaze at an electrical substation in Hayes, west London.
Homes have been left without power, with videos on social media showing thick plumes of smoke.
London Fire Brigade (LFB) said 10 engines and around 70 firefighters were at the scene on Nestles Avenue.
Around 150 people have been evacuated and a 200-metre cordon has been put in place.
The cause is so far unknown.
LFB said it had received more than 190 calls over the incident – with the first received at 11.23pm.
Residents have been urged to keep windows and doors closed due to the “significant amount of smoke”.
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Scottish and Southern Electricity Networks said on X there was a large power outage in the area.
“We’re aware of a widespread power cut affecting many of our customers around the Hayes, Hounslow and surrounding areas,” they said.
A graphic on the company’s website suggested around 16,000 customers could be affected.
It estimated the issue would be resolved by 3am.
Firefighters from Heathrow Airport, Hayes, Hillingdon, Southall and other stations are all involved in the response.
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