The companies are permitted to spill excess wastewater and rainwater into waterways during intense downpours, through storm overflow discharges, to prevent sewage systems from becoming overloaded and backing up into homes and streets.

But Surfers Against Sewage said it detected 146 dry spills, when there was no rain, between October 2021 and September 2022.

Over that period, the campaign group issued 9,216 sewage pollution alerts through its Safer Seas & Rivers Service (SSRS), which covers over 450 beach and river spots across the UK.

A representative from Surfers against Sewage protests against sewage discharges at an overflow pipe on Long Rock Beach in Penzance, Cornwall

However, Water companies state dry spills are not always illegal. They are permitted to treat a certain amount of wastewater per second and if it exceeds that level, they can discharge it.

That level can be affected by groundwater infiltration, sewer blockages, pump failures and drainage times in each area, as well as rainfall.

Amy Slack, Head of Campaigns at SAS, said: “Over the last year, the UK public has made clear their disgust at what’s happening to our rivers and seas, and yet water companies continue to pollute at will.

“It’s especially alarming to uncover evidence of potentially illegal activity by water companies in the form of ‘dry spills’, which are not permitted under current regulations. Shareholders and CEOs are unashamedly profiteering off pollution.”

Surfers Against Sewage (SAS) said it detected 146 dry spills, when there was no rain, between October 2021 and September 2022.

Waters UK, which represents water companies, said they are planning to significantly reduce storm overflow discharges by investing £56bn in “one of the country’s largest ever infrastructure programmes”.

A spokeswoman added: “To accelerate progress further, we need Government to end housing developers’ uncontrolled connections to sewers without first knowing their capacity, and to end the flushing of wet wipes made from materials that cause blockages and fatbergs.

“Both are major causes of sewer overloading and spills."

England’s nine water companies have been heavily criticised for pumping sewage into the country’s waterways and urged to minimise pollution in recent months.

Environment Agency Chair Emma Boyd recently criticised their “shocking” performance, after 62 serious pollution incidents were recorded last year – the highest total since 2013.

She said company directors have “let this occur” and they have been “rewarded handsomely” while the environment “pays the price.”

The Environment Agency also stated Yorkshire Water’s environmental performance requires “significant improvement”, as it recorded 74 pollution incidents last year and five were deemed to be serious.

It discharged raw sewage into rivers or the sea more than 70,000 times in 2021, figures from the Environment Agency show.

However, the companies claim they have to discharge some excess wastewater and rainwater during heavy storms, to prevent sewage systems from becoming overloaded.

That is because the original sewer system, designed by the Victorians, has one pipe for wastewater and rainwater. The Government claims it would cost between £350bn and £600bn to replace that system.

Offwatt, which regulates the water companies, said investment in the industry has roughly doubled since privatisation in 1989, and companies spent around £6bn a year on upgrading their infrastructure between 2015 and 2020.

But they also paid out £57bn in dividends to shareholders and parent companies between 1991 and 2019.

Water firms accused of releasing sewage into rivers and sea during dry weather