Assistant commissioner Andy Pennick said the past 24 hours had been “incredibly challenging” for firefighters and control staff, who had faced “arduous, hot conditions”.
“In these hot, dry conditions, grass fires can spread incredibly quickly, threatening homes, wildlife and the environment, while tying up vital emergency resources,” he said.
Pennick urged people not to use disposable barbecues in parks or open spaces, to dispose of cigarettes responsibly and to take litter home if bins were full.
He said home barbecues should be kept well away from fences, sheds and trees, and advised clearing dry vegetation from around buildings.
The warning comes as firefighters attended two more grass fires in the capital.
Eight fire engines and about 60 firefighters were called to a blaze near South End Road in Hornchurch at 13:18.
At its height, about six hectares (15 acres) of grassland were alight. No injuries were reported.
Later, eight fire engines and another 60 firefighters were sent to a grass fire near Berwick Pond Road in Rainham.
The brigade said it received the first of 11 emergency calls at 15:25 and brought the blaze under control by 17:12.
About six hectares of grassland also burned in Rainham.
The cause of both fires is not yet known.
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