Nearly 40 beach pollution warnings have been issued in 48 hours as water companies poured sewage into rivers and the sea.
Heavy rainfall on Tuesday and Wednesday led to mass sewage dumps into the sea around the south coast of England, leading to warnings against bathing because of bacteria levels.
The Environment Agency warned people not to swim at 17 bathing water sites in the south-west of England, while the charity Surfers Against Sewage highlighted sewage spills or pollution at 37 sites in total across the south coast.
It came as Thames Water on Wednesday became the latest company to announce a hosepipe ban, which will come into force for its 10 million customers from Aug 24.
The move will more than triple the number of people under restrictions following similar announcements from South West Water, South East Water, Southern Water and Welsh Water.
Yorkshire Water will bring in restrictions for its five million customers from Aug 26.
The storms are not expected to outweigh the effects of weeks of dry weather this summer, which have led to drought conditions across the south and east of England, as well as in Yorkshire.
Heavy rainfall leads to sewage releases as systems designed to combine rain and sewage become overwhelmed with rainwater, meaning they do not treat the sewage and release it raw into the environment.
Raw sewage pumped into sea today at Seaford, E. Sussex after storm. Yep…that’s a Marine Protected Area. 1000s swimming here just yesterday. Horrendous. @sascampaigns @SouthernWater @mariacaulfield @sussex_bythesea pic.twitter.com/93QjK6Ib5r
— martyn craddock (@craddock1970) August 16, 2022
Water companies are only supposed to do this in "exceptional" circumstances, according to a 2012 European Court of Justice ruling.
But in practice, it happens thousands of times a year because of historic underinvestment, heavy bursts of rainfall, population growth and increased runoff from housing, paved gardens and roads.
In a statement on its website, Wessex Water said storm overflows "prevent properties from flooding following intense rainfall" and should "have minimal or no ecological impact because what is released is diluted wastewater".
Flash flooding hit the Midlands and south on Tuesday and Wednesday, with homes and businesses flooded in towns such as Market Rasen, in Lincolnshire, and Worksop, in Nottinghamshire.
Rivers are already extremely low and lacking in oxygen because of the drought - and sewage is likely to make an already dire situation worse.
Stuart Singleton-White, head of campaigns at the Angling Trust, said: "Normally a storm overflow should work when there is heavy rain, and there is high river flow, and it is washed out into the system, and if it's only temporary the impacts are usually minimal.
"With the low river flows, with the historic pollution, and where so many of our rivers are losing important vertebrates and plant life because of sewage sludge, phosphate and ammonia, this sewage is only going to make all of that worse.
"A summer of low flows, a summer of over-abstraction, a bit of rain comes back and now we get sewage poured into these rivers which are already struggling."
Hugo Tagholm, chief executive of Surfers Against Sewage, said on Wednesday: "England’s water companies seem to have failed to plan both for too little and too much water as we’ve seen in recent days.
"Yesterday saw beaches across the south coast and Cornwall posing real health risks to swimmers and surfers due to dozens of sewage discharges at some of the country’s favourite beaches.
"This is becoming all too familiar standard practice from water companies when it rains, a little or a lot. Last year, we tracked almost 3,500 sewage pollution at beaches, including some flying the Blue Flag."
A spokesman for the Environment Agency said on Wednesday: "Throughout the bathing season, daily pollution risk forecasts are made for a number of bathing waters where water quality may be temporarily reduced due to factors, such as heavy rainfall - as is the case today - wind or the tide.
"When the potential for a temporary reduction in water quality is forecast, we issue a pollution risk warning and advice against bathing.
"This enables bathers to make informed decisions regarding avoiding times or locations where the risk of pollution is higher than normal, and health risks from bathing may be higher than the annual classification suggests."
Flash floods and thunderstorms are forecast to hit parts of the UK over the coming days, with the Met Office issuing an amber thunderstorm warning for areas of Cornwall, Devon and Somerset - presenting a danger of homes and businesses flooding, power cuts and transport chaos.
Victoria station is flooding… pic.twitter.com/0fdomrqMew
— Anthony Bonnici (@Anthony_Bonnici) August 17, 2022
The Met Office has issued an amber alert for thunderstorms on Wednesday covering a swathe of England from Chichester to Ipswich, and including London and all of Kent.
The warning, which runs from 11am to 10pm, said flooding of homes and businesses is "likely", with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
Fast flowing or deep floodwater is also said to be "likely, causing danger to life".