Raw sewage was discharged into rivers and coastal waters in England for almost 4m hours last year, with waterways that have the highest environmental protections subjected to days of pollution.

Data released by the Environment Agency on Thursday revealed water companies discharged untreated effluent for 3.62m hours, a slight increase on last year.

The data showed that nothing had changed, Giles Bristow from Surfers against Sewage said. “The picture is just as bleak. Despite commitments to investment over the past five years, our water is no cleaner.”

The increase in the hours of raw sewage dumped into waterways comes as water companies are increasing bills by an average of £123 to pay for replacement and upgrading of ageing infrastructure, in the biggest hit to customers’ pockets since the industry was privatised 36 years ago.

The environment secretary, Steve Reed, said: “These figures are disgraceful and are a stark reminder of how years of underinvestment have led to water companies discharging unacceptable levels of sewage into our rivers, lakes, and seas.”

While water companies said the number of incidents of discharges had reduced by 2% to 450,398, the duration of raw sewage poured into waterways increased again, to 3.62m hours.

Some companies were responsible for huge rises in raw sewage dumping. Anglian Water was behind the biggest increase, discharging 448,938 hours of raw sewage, a 64% increase on 2023.

As revealed by the Guardian, Thames Water increased raw sewage discharges by more than 50%, to almost 300,000 hours.

Rivers and beaches that have the highest environmental protections were subjected to significant sewage discharges. According to analysis of the data by Peter Hammond, a mathematician who monitors sewage discharges, in February and March last year Wessex Water discharged raw sewage for 36 days at Chesil Beach in Dorset. The beach has some of the highest environmental protections, with special area of conservation, site of special scientific interest and Ramsar (also known as the Convention on Wetlands) status.

Northumbrian Water discharged raw sewage for 67 days between January and March from the Norham treatment works into the River Tweed, a waterway that has SAC and SSSI status, according to analysis of the data.

South West Water discharged untreated effluent for 107 days between January and April last year from the Salcombe Regis sewage treatment works into the beach at Salcombe Mouth, a popular tourist area.

Bristow said: “There is zero justification for sewage pollution at this sickening scale and the bill rises coming in next week are simply forcing a cash-strapped public to throw good money after bad.

“The simple truth is our water industry is broken, the public’s trust is long gone, and the only course left is radical reform and the end of profit from pollution.”

Storm overflows, which release raw sewage from treatment plants and elsewhere along the system, were designed to be used only in extraordinary circumstances to relieve pressure in the system. But a combination of a lack of investment by water firms and higher rainfall caused by the climate crisis, mean these overflows are being routinely used across the industry. The data released by the EA shows stop-start times for raw sewage discharges monitored by event duration monitors on 14,285 storm overflows.

The government, in its storm overflow reduction plan, has a target for the average spills for each overflow to be reduced to 10 by 2050. But most companies have much higher average spills, ranging from Severn Trent at 25 to South West Water at 41.

Some companies did significantly reduce their sewage discharges. United Utilities reported a 31% reduction, to 450,178 hours, and Southern Water had a slight reduction of 4% in hours of discharge, falling to 304,537. Yorkshire Water also cut its discharges.

Reed said Labour had imposed tough measures on water companies, banning unfair bonuses for polluting water bosses and introducing criminal charges for lawbreakers. “But we will go further and faster,” he said. “That’s why this government has secured over £100bn of private sector investment to upgrade our crumbling infrastructure.”

Clive Lewis, the MP for Norwich South, whose private member bill on the future of water management will be debated in parliament on Friday said: “Water should be considered a national security issue, yet toothless regulation and a debt-ridden private sector monopoly makes it a playground for corporate greed.”

A Wessex Water spokesperson said: “We’ve just completed a £1m project that will stop discharges from an overflow at Chesil Beach, increasing sewer capacity to redirect flows of rain and foul water to a larger sewer nearby for onward treatment.

“We agree that overflows are outdated and we’re spending £3m every month on schemes to reduce how often they operate. This investment will massively increase in the coming years and must be supported by environmental policymaking that promotes better rainwater management at source.”

A Water UK spokesperson said: “While these results are disappointing, we are starting to see the effect of investment with many companies reducing the number of spills in their area, despite 2024 being one of the wettest years on record.

“From April water companies will invest £12bn to almost halve spills from storm overflows by 2030. This is part of the largest amount of money ever spent on the natural environment to help support economic growth, build more homes, secure our water supplies and end sewage entering our rivers and seas.”