Southern Water slammed for "illegal" dry spills running into hundreds of hours over the past year
Water companies are only supposed to spill after heavy rain to prevent the system from backing up and flooding homes and businesses with raw sewage.
Other water companies declined to give their data to the BBC because they are already under a criminal investigation by the Environment Agency.
The three companies reportedly collectively released sewage in dry spills for 3,500 hours in 2022 – in breach of their permits. Southern Water discharged sewage 63 times in 2022 for a total of 792 hours - or the equivalent of 33 continuous days.
Discharging in dry conditions is illegal under environmental law, as releasing sewage without rainwater is more likely to result in a buildup of toxin producing algae which poses health risks for swimmers and pets.
The BBC investigation reports that Southern, Thames and Wessex illegally started releasing sewage on dry days 388 times last year. This includes 19 July 2022, the hottest day on record, when temperatures topped 40C in some places with many people cooling off in bathing waters.
Portsmouth South MP Stephen Morgan said: “The Conservatives have allowed oceans and seas to be flooded with sewage, not only damaging Portsmouth’s precious environment but also our ability to enjoy the coast without fear of our health and safety.
"Southern Water are acting without regard for the environment and public health and breaking the law in doing so. Tory MPs must explain why they have so far voted against taking action against holding water companies accountable.
"In government, Labour will introduce automatic fines to punish law-breaking companies, make water bosses responsible for their negligence and bring in strict mandatory monitoring of outlets to make sure this never happens again”.
Steve Reed MP, Labour’s new shadow environment secretary, added: “There can be no more damning metaphor for 13 years of Tory failure than the stinking, toxic sewage now polluting our rivers and lapping onto our beaches. This scandal is their fault. They cut back enforcement and monitoring against water companies releasing this filth and are now failing to prosecute them when they are blatantly breaking the law.
"There must be an immediate investigation into both the breach of the license and the environmental damage caused. Only then can we expose this illegal pollution and bring those responsible to justice".
John Penicud, head of wastewater operations at Southern Water, told the BBC that "so-called 'dry spills' are a complex issue" and said discharges in dry weather can be caused by groundwater entering pipes.”
Referring to one particular incident highlighted at the River Lavant, near Chichester, added: "Lavant is in a catchment that is particularly prone to groundwater infiltration."
He also said that the company planned to invest more than £1.6m improving more than 2.5 miles of sewers in the area.