Forecasters said thunderstorms were the “wrong kind of rain” — despite the need to tackle drought conditions — as the sun-hardened ground would be unable to absorb a deluge.
Storm clouds were heading north from the Channel and were expected to hit London by early afternoon, becoming more extensive into the evening.
Torrential rain was expected to bring disruption. “Heavy downpours may cause localised flooding and travel disruption,” the Met Office said. The yellow warning is also in place for tomorrow, with heavy rain forecast in the capital and across the South-East.
There were isolated brief showers in London on Monday but a threatened downpour failed to materialise.
Temperatures in London are due to fall below 20C on Tuesday evening as the rain arrives. The concern is that with the ground bone dry after weeks of hot weather, natural drainage will be impossible and areas will quickly become flooded.
Experts said people who live in “low-lying properties” should ensure valuable items were “ready to go” due to the current high flood risk.
Met Office meteorologist Clare Nasir told Sky News: “For low-lying properties, which perhaps have been built on a floodplain, yes, there is a risk of flooding in properties. Get all your documents, whether it’s your mobile phone, your passport, all those things you don’t want to be damaged by floodwater and make sure they’re ready to go or on a higher level of your house.”
She added: “What we’re looking for is sort of continuous rain, moderate rain, rather than this incredibly intense burst which currently is moving up across more southern areas of England. So we’re not out of the woods yet.”
The Met Office said lightning, hail or strong winds could lead to rail cancellations and make the roads hazardous. Fast-flowing floodwater presented a risk to life, it added.
Monday was the eighth consecutive day where temperatures above 30C were recorded in the UK. London Fire Brigade received almost 3,400 emergency calls over the weekend and attended more than 200 grass fires, including in Epping Forest and in Enfield.
LFB assistant commissioner Jonathan Smith said: “Londoners also need to prepare themselves for the potential risk of flash flooding, as the ground is so dry even a small amount of rainfall could result in a flash flood.”
Meanwhile, the head of Ofwat has defended water regulation, saying targets for areas such as leaks are “challenging but achievable”.
David Black, head of the water regulator for England and Wales, said he does have concerns about the performance of some companies, but also said there “isn’t sufficient account given” for what is happening in the sector.
The Ofwat chief executive’s comments come amid renewed scrutiny of water companies during a period of drought and with some areas of the country facing hosepipe bans.
Speaking to BBC Radio 4’s Today programme and asked if the targets set for leaks are tough enough, Mr Black said: “Yes, we set challenging but achievable targets for water companies, leakage being one of them.”