London Fire Brigade Assistant Commissioner Jonathan Smith said water supplies are being monitored “very closely” as temperatures soar to 32C in the capital and the city faces a hosepipe ban.
He issued an urgent appeal after new data showed an eightfold increase in blazes in the first week of August alone.
LFB said thousands of firefighters had fought 340 wildfires in the first week of August, compared to just 42 in the same week last year.
“We want the public to assess their behaviour, particularly over the course of the next four days," Mr Smith told the BBC.
"We want to try to avoid repeats of the scenes that we saw on the July 19 when people lost their homes and their livelihoods. "Prevention is better than cure."
He added: “The water supply is something that we keep a very, very close eye on because we need that in terms of our firefighting activity.
"So we're thinking about how we make sure that we maximise the water that we carry on our firefighting appliances. And it's really where this partnership needs to come in between emergency services, the water companies and the general public to make sure that we're conserving water as best we can and trying to prevent these fires from occurring in the first place."
It comes as officials at the Met Office also warned that a lack of rainfall means grass is extremely dry, meaning there is an “exceptional” fire risk for parts of London from Friday.
Mr Smith said: “This summer has seen an unprecedented long, dry spell with high temperatures so the grass in London is tinderbox dry and the smallest of sparks can start a blaze which could cause devastation.
“Despite our continued warning over the last few weeks, we know there are still people who are barbecuing in parks, dropping cigarettes out of car windows and leaving rubbish lying around.”
Riccardo La Torre, national officer for the Fire Brigades Union, warned services across the UK are "completely unprepared" for the level of risk posed by the imminent heatwave.
"These are brutal, brutal fires to fight," he told Sky News. The temperature that they burn at, the speed at which they spread at.
"The reality is we've been left completely unprepared to do that as a fire and rescue service.
"We've had over a fifth of the workforce cut since 2010, that's over 11,500 firefighters cut. Yet we're asking them to deal with these extreme weather events in increasing regularity and increasing severity."
July’s heatwave saw devastating blazes which torched homes across the capital, and resulted in the Brigade’s busiest day since the Second World War.
Among them was an inferno in the village of Wennington, east London, which destroyed multiple properties.
The Brigade said calls had also significantly increased overall, with 18,603 calls between July 18 and August 7, compared to 12,102 calls over the same period last year.
The warning comes as the capital is set to face a hosepipe ban, with Thames Water saying it will introduce the measure in the coming weeks.
Network Rail has also advised that commuters could see localised speed restrictions on trains as temperatures soar during the Level Three heatwave.
However, a spokesperson has told the Standard there are no “blanket speed restrictions” planned for this week.