Liz Truss slashed funding to tackle water pollution when she was Environment Secretary which led to "doubled sewage discharge", Labour has claimed.
The Tory leadership frontrunner was accused of having "sewage on her hands" after analysis showed raw sewage discharge in England and Wales more than doubled from 14.7 per overflow in 2016 to 29.3 in 2021.
Ms Truss ran the Department for Environment, Farming and Rural Affairs (Defra) from 2014 to 2016, where she oversaw efforts to slash £235million from the Environment Agency's budget.
This included £80m of cuts to sewage monitors for the agency, which works with water companies to ensure they are closely monitoring and reporting sewage discharge activity.
Vaughan Lewis, a senior consultant for the Environment Agency told the Guardian that cuts made it "impossible" for staff to properly monitor the issue.
It comes amid mounting anger over the dumping of raw sewage into rivers and seas, as dozens of pollution warnings were issued for beaches and swimming spots following heavy rain that overwhelmed the sewage system.
As Environment Secretary, Ms Truss justified the cuts saying "there are ways we can make savings as a department" citing better use of technology and inter-agency working.
But Shadow Environment Secretary Jim McMahon said: "Under the Tories, the country is facing a crisis in our water supply.
"Our water infrastructure is at bursting point, with billions of litres of water being wasted every day and raw sewage being dumped into our waters.
"The fact that Liz Truss was the one to cut the EA so severely, not only demonstrates her lack of foresight but also her lack of care for the detail, in recognising the need to adapt to the serious flooding that had just happened on her watch."
Vaughan Lewis, a senior consultant for the Environment Agency told the Guardian: "They [the cuts] plummeted to the point it was impossible for the Environment Agency to know what's going on.
"They had no control or monitoring capability that was meaningful. They ceded the control of monitoring to water companies, which ended up being able to mark their own homework.
"They take their own samples and assess whether they are being compliant."
He warned citizen scientists were being left to monitor and fill the gaps.
Mr Lewis added: "Lots of this would have happened under Liz Truss; she was there when some of those cuts were made.
"She was a poor minister and the Environment Agency has been cut to the bone, and it can't monitor or regulate effectively."
Greenpeace UK’s chief scientist, Dr Doug Parr, said: “A decade of budget cuts and government deregulation has left the Environment Agency, almost literally, up shit creek without a paddle.
"The growing tsunami of sewage unleashed on to Britain’s waterways is a shocking demonstration of how undermining our regulators leads to a disregard for nature and those meant to protect it.
“That our likely future prime minister was an instigator of cuts to the money used to protect our rivers, and so helped cause this environmental catastrophe, doesn’t bode well for the UK’s protection of the natural world. Liz Truss has sewage on her hands."
A spokeswoman for Liz Truss said: "These spending reductions were part of a wider drive from central government to find efficiencies across department budgets and government agencies.
"It’s vital we get a grip on pollution in our water and ensure it is clean and safe for all to enjoy. As Prime Minister, Liz will make sure the necessary action is taken to deliver this."
There has been growing public anger at the volume of raw or partially treated sewage pumped into the UK's rivers and coastal waters.
Water firms are being criticised for not investing money back into the UK's outdated water infrastructure, with mounting pressure on ministers to intervene.
No10 said on Monday that it was the duty of firms to put customers before shareholders.
A spokeswoman for the PM said: "We have been clear that the failure of water companies to adequately reduce sewage discharges is completely unacceptable.
"They have a duty to put their customers before shareholders and we would expect them to take urgent action on this issue or face fines."
The spokeswoman added: "We continue to speak regularly with them. The Environment Agency undertake enforcement action and monitoring, which we have stepped up."
Downing Street also said water companies were already facing legal action from regulators.
The spokeswoman said: "Since 2015 the Environment Agency has brought 48 prosecutions against water and sewerage companies, securing fines of over £137 million."
She added that since privatisation, the equivalent of £5 billion had been invested to upgrade water infrastructure, but the companies must "continue to take action".
