An immediate investigation must be launched into reports that three major water companies illegally discharged sewage hundreds of times last year, ministers were told on Tuesday.
The practice involves sewage being pumped into waterways during dry conditions and is illegal under environmental law.
Releasing sewage into rivers and seas is allowed to prevent pipe systems becoming overwhelmed or blocked, but only when it is raining.
Thames, Wessex and Southern Water discharged waste for a total of 3,500 hours in 2022 when the weather was dry, a BBC investigation found.
Labour’s new shadow environment secretary Steve Reed said: “There must be an immediate investigation into both the breach of the licence and the environmental damage caused.
“Only then can we expose this illegal pollution and bring those responsible to justice.
“Labour would 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.”
Discharges when it is not raining mean the sewage is less likely to be diluted and can lead to build-ups of algae that produces toxins dangerous to pets and swimmers.
Liberal Democrat MP Tim Farron said: “These revelations are scandalous and the Government must act immediately.
“Ministers have been told for months about these dry spills yet they refuse to take any action.
“These companies should be held criminally responsible and see their day in court if found to be breaching their permits.”
Thames, Southern and Wessex started illegally releasing sewage on dry days 388 times, research by the BBC’s climate and data team found.
Releases were even recorded when the regions were in drought and on July 19, 2022, the hottest day on record in Britain, when temperatures topped 40C in some areas.
Environment Secretary Thérèse Coffey told the BBC: “It does seem extraordinary on the hottest day of the year that there may be releases. The Environment Agency is the regulator, they are the people who do the detailed investigation of why that has happened.”
Thames Water’s Longbridge Road overflow site in Dagenham saw multiple spills for nearly 200 hours, a quarter of which were estimated to be during dry conditions.
A Thames Water spokesman said: “We regard all discharges of untreated sewage as unacceptable, and we have planned investment in our sewage treatment works to reduce the need for untreated discharges including at Stewkley, Stone and Haddenham.
“We are the first water company to provide storm overflow alerts for inland waters and this ‘near real-time’ data is available to customers as a map on our website. We want to lead the way with this transparent approach to data and we have also published on our website our plans to upgrade over 250 of our sewage treatment works and sites.
“Stopping discharges altogether will take time and sustained investment, however each step we take on this journey is a move in the right direction.”
John Penicud, head of wastewater at Southern Water, said: “So called ‘dry spills’ are a complex issue. Water is a powerful force of nature – and high groundwater conditions can lead to rising water finding the path of least resistance into a network of sewer pipes and manholes, and a discharge made up of groundwater is not caused by rainfall and can happen in dry weather. It is required to be reported as a ‘spill’.
“The problem is especially challenging in areas prone to flooding, as mitigation measures such as sewer relining and manhole sealing redirect flows and groundwater can then cause flooding. Private, illegal connections to the system are another potential source.
“We work with the Environment Agency and stakeholders to cut these so-called ‘dry spills’ – and all forms of water and wastewater releases.”
A Wessex Water spokesman said: “This a known issue caused by high groundwater which, unlike rainfall that stops relatively quickly, continues for days or even months. None of these overflows cause rivers to fail to meet ecological standards.
“We’re using nature-based solutions to treat groundwater affected sites, and by 2025 we will have completed or progressed 28 schemes in our region.
“Meanwhile we’re investing £3m a month on reducing overflows, which we agree aren’t fit for the 21st century, and we’re planning to invest significantly more between 2025 and 2030 subject to regulatory approval.”
An Environment Agency spokesman said: “We are conducting our largest ever criminal investigation into potential widespread non-compliance by water and sewerage companies at thousands of sewage treatment works. Our tough enforcement action has already led to over £150m in fines since 2015.
We will always pursue and prosecute companies that are deliberately obstructive or misleading – and work constructively with those driving improvements.
We are also improving how we regulate the sector – including expanding the number of officers focused solely on regulation, increasing compliance checks and recruiting more data specialists able to translate storm overflows monitoring data into stronger regulatory intelligence”