A 'risk to life' warning has been issued and severe flooding is "likely" for parts of the UK which could see a month's worth of rain in just a few hours.
The Met Office upgraded its rain warning from yellow to amber – the second-highest warning level – for London and parts of south-east England, saying 50-60mm could fall in just four to six hours.
Forecasters warned that flooding of homes and roads, and travel disruption from heavy rain are likely Monday afternoon and evening.
Fast flowing floodwater is possible, causing danger to life, along with power cuts and loss of other services to homes and businesses, the Met Office said.
The Met Office said: "Following earlier rainfall some heavy, and perhaps thundery, showers are expected to develop over parts of south-east England this afternoon and into the evening.
"Whilst many areas will miss the highest rainfall totals from these some places could see 50-60mm falling in the space of four to six hours.
"The showers will then gradually ease later this evening."
The amber warning has been issued for Berkshire, Buckinghamshire, East Sussex, Essex, Greater London, Hampshire, Hertfordshire, Kent, Oxfordshire, Surrey and West Sussex.
A yellow warning has been issued for a wider area, extending from the Humber Estuary south to the English Channel, and west to places including Sheffield, Derby, Coventry, Oxford, Winchester and Portsmouth.
That warning reads: "Areas of, at times heavy, rain will push north and then westwards across much of south and east England on Monday.
"Whilst some places will miss the worst of the rain 10-25mm of rain may occur quite widely, with some areas potentially seeing up to 60mm and much of this falling in less than six hours.
"There is also a chance that thunder may develop later in the day over parts of East Anglia and the far south-east of England."
A separate yellow warning was issued on Monday afternoong for Devon, Cornwall and parts of South Wales, including Cardiff and Swansea.
The Met Office said: "Heavy and slow moving showers have formed over south-west England and south Wales.
"These are generating some large rainfall totals with 15-20mm in an hour and 30mm in two to three hours.
"The showers should start moving more quickly later this afternoon but some impacts may persist into the busy travel period."
The heaviest rain overnight was recorded in Sellindge, Kent, where 0.6in (15mm) fell in 12 hours, the Met Office said.
Forecaster Steven Keates said the northern Home Counties, East Midlands, Lincolnshire and Humber could see some of the heaviest and most persistent rain.
"Probably later this afternoon into this evening and tonight we could see some of the heaviest rain," he said.
"It is going to be more the persistence of the rain in some places and the intensity for others.
"Eastern and southern England in particular are likely to be seeing long and persistent bad weather.
"Some places could see a month's worth of rain in two or three days, and we could see some flooding issues."
The Environment Agency (EA) has one flood alert on the River Ravensbourne in London covering the boroughs of Lewisham, Bromley, Greenwich and Croydon.
It said there was a risk of flooding for low-lying land, gardens and roads that are close to the river between 7am and 6pm.
The EA added: "We are actively monitoring the situation and are prepared to respond if required.
"Please avoid using low-lying footpaths near local watercourses and plan driving routes to avoid low-lying roads near rivers, which may be flooded."
The band of rain will move slightly north and west before settling, with Northern Ireland and Scotland expected to see the best of the weather.
Weather warnings are in place for the whole of Tuesday and up until midday on both Wednesday and Thursday for north-eastern England down to the Wash and as far west as the West Midlands.
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