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.
LFB said it had rescued people trapped in cars and 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 is in place for parts of Kent and Sussex between on Monday.
A spokesperson for Barts Health NHS Trust which runs Whipps Cross hospital 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.”
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.
On Tuesday and Wednesday a yellow storm warning has been issued for much of the Midlands and northern England.
Meanwhile, a yellow thunderstorm warning is in place for most of Scotland between for 12 hours from noon on Tuesday with heavy rain expected until Thursday morning.
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.
Residents on a street in Woodford, in east London, grabbed buckets, brooms and wooden boards to prevent rising rainwater from flooding their homes.
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Resident Chris Date tweeted a photo of a bus in Bakers Arms, Walthamstow, saying the flood water was “above the kerb”.
“It’s impossible to walk on the pavement. To get on that bus the water came up to my shins. This is a canal, not a road,” he said.
Mr Elliott said the flooding was the worst he had experienced in the city, and described seeing “buses stood broken down in the water”.
The Mayor of London, Sadiq Khan said he was “in contact with key partners” with whom he will hold a roundtable.
Mr Khan said: “We are seeing increasing incidents of extreme weather events linked to climate change. This is not the first time in recent weeks that London has been hit by major flooding.
“Despite having limited powers in the area, it remains a key priority for myself and London’s council leaders that more is done urgently to tackle flooding and the other impacts of climate change.”
‘Victorian era drainage’
Dr Veronica Edmonds-Brown, senior lecturer in aquatic ecology at the University of Hertfordshire said London’s Victorian era drainage system “cannot cope with the huge increase in population”.
Dr Edmonds-Brown said there were several reasons for Sunday’s flooding.
“The first is building on the floodplains of the Thames and River Lee. The second is urbanisation. The more impervious surfaces we install – and we are amid a rapid housing programme at the moment – the worse this situation will get.
“The final reason is that our drainage system is not built for the amount of water it is receiving. Due to budget limitations, local authorities are not able to maintain or improve them.”