More than half a month's worth of rain could fall in just two to three hours as severe and slow-moving thunderstorms hammer parts of the UK.
The Met Office said storms threaten to flood homes and roads, cause damage from lightning strikes or hail, cut power and disrupt travel.
The forecaster has issued a warning for severe thunderstorms which are expected to linger for almost 10 hours on Monday in parts of south and south-east England, including London.
About 25mm of rain could fall in just one hour in places, while the heaviest downpours could drop 40-60mm in two to three hours.
Thunderstorms are striking just days after most of Britain was battered by strong winds and heavy rain, and a power cut caused chaos in England and Wales.
The Met Office said: It said: "Heavy showers and thunderstorms are already affecting some coastal areas, and are expected to develop elsewhere across south-east England later this morning and through the afternoon.
"While some places are likely to remain dry, slow-moving thunderstorms could give around 25mm of rain in one hour and perhaps 40-60mm of rain in two to three hours in a few places.
"Lightning and hail are also possible."
The Met Office said the main threats include:
- A small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes or hail.
- Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
- Spray and sudden flooding could lead to difficult driving conditions and some road closures.
- There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost.
The warning is in place from 10am until 7pm.
More than 70mm of rain fell in Cumbria on Saturday as the country was struck by unseasonable heavy rain and strong winds.
The Met Office said the wettest part of the country was Spadeadam, where 71.4mm fell on Saturday - compared to a monthly average of 82.4mm for August in the region.
On Saturday, flooded tracks forced train operators to cancel services between Cumbria and Scotland in a day of disruption on the rail network.
The strongest wind speeds were recorded in Mumbles Head in South Wales where gusts reach 64mph on Saturday morning, followed by 58mph recorded in Aberdaron and 56mph at Pembrey Sands.
On Friday, one of the worst power cuts in Britain in years caused travel mayhem and an electricity outage for almost a million people in England and Wales.
It was caused by simultaneous failures at a gas-fired power plant in Bedfordshire and an offshore wind farm in East Yorkshire.
