A hospital has been forced to evacuate patients and cancel operations after heavy rainfall caused it to lose power.
Around 100 inpatients at Whipps Cross Hospital in east London have been moved elsewhere and ambulances diverted after the NHS declared a “major incident”.
Newham Hospital’s A&E department had also urged people to seek urgent medical care elsewhere.
London Fire Brigade (LFB) said it had taken more than 1,000 flooding-related calls across London.
LFB said it had rescued people trapped in cars and also helped with flooded basements and collapsed ceilings.
The Met Office has issued further warnings over rain and thunderstorms in England and Scotland over the next four days.
A yellow warning has been issued for parts of Kent and Sussex on Monday.
A spokesperson for Barts Health NHS Trust said the situation was being kept “under constant review” after cancelling all planned surgery and outpatient appointments for the day.
They added: “We are diverting ambulances while we work hard to clean up affected areas of the hospital.
“The emergency department at the hospital remains open for walk-ins but patients requiring urgent treatment are asked to attend alternative hospitals where possible.”
Newham Hospital is now fully operational after also being affected by flooding.
Stepney Green Tube station remains closed although eight other Tube and London Overground stations have reopened.
St James’s Park in London saw 41.6mm (1.6in) of rain on Sunday, making it the wettest part of the country.
Barking in east London reported 41.3mm on Sunday, most of which fell between 15.00 BST and 17.00.
This has been the second 40mm rainfall event in London this month.
Many of the capital’s roads closed due to the flooding, including the Blackwall Tunnel, the A12 and parts of the North Circular.
Standon Calling festival in Hertfordshire was abandoned due to Sunday’s flooding.
One flood warning for the Isle of Wight, and a further 10 less severe flood alerts have also been issued, covering most of the capital and parts of surrounding counties.
A yellow storm warning has also been issued for Tuesday and Wednesday for much of the Midlands and northern England.
Across most of Scotland a yellow thunderstorm warning has been put in place from noon on Tuesday with heavy rain expected until Thursday morning.