Water industry workers say they have been physically assaulted and feel unsafe working alone for fear of attack amid a public backlash over sewage dumping.

More than one in three UK water employees have been verbally abused at work, according to a survey of almost 1,300 staff conducted by the GMB union.

Water companies in England faced a barrage of criticism last week as data revealed that raw sewage was discharged for more than 3.6m hours into rivers and seas last year – the worst year on record for storm water pollution.

The companies – including heavily indebted Thames Water which faces questions over its financial future after investors said on Thursday they were unwilling to invest further funds – have been widely condemned by the public and politicians over leaks and spills.

That anger appears to have manifested on the frontline. Workers complained that they had faced “very hostile” abuse from the public, in a series of anonymous testimonies delivered to the GMB and shared exclusively with the Guardian.

One employee said they had stopped interacting with members of the public after a colleague was “violently assaulted and was off work for weeks with a broken jaw” and another claimed an employee was attacked with a machete.

Another said they had been “physically assaulted by minors while shutting down a vandalised fire hydrant while their parents stood laughing”.

An employee, who now refuses to wear clothing with the company logo, wrote of an alleged incident where colleagues were dealing with a sewage spill in a road. “Drivers were purposefully splashing staff with sewage. He was soaked. It was in his eyes and face. Appalling and disgusting behaviour by the public,” they said.

Another worker said they now “feel unsafe working alone in a lot more places and areas than I used to”.

According to the survey, 52% of workers said they believed reports of sewage dumping had contributed to an increase in abuse.

Workers said they felt “underpaid and underappreciated” and that abuse frequently came from motorists, when works enforced road closures. Call centre workers had also suffered abuse, they said.

The GMB research found that just 20% of wastewater workers thought infrastructure was good enough to stop leaks, and the same number had witnessed unreported sewage spills in the past year. One in five said they had either personally been encouraged to under-report sewage spills, or knew a colleague who had.

One respondent said managers had not investigated reports of abusive customer behaviour as they were “afraid of customer complaints affecting the score card targets. It is easy for managers not to investigate aggressive customer behaviour.”

Gary Carter, GMB national officer, said: “No one should go to work and face abuse. But this situation is horrifying, because the negligent actions of water bosses in allowing sewage dumping to rocket has exposed their own workers to physical and verbal violence.

“GMB demands a zero tolerance approach to the abuse of water workers – and calls on water shareholders to fork out for the desperately need infrastructure to stop record sewage spills.”

Last week the Environment Agency launched a portal to make it easier for water company whistleblowers to safely report serious environmental wrongdoing by their employer.

The agency said it was a “bid to crackdown on sewage pollution and other environmental wrongdoing” and said any findings could be used to support enforcement action against companies, including financial penalties and criminal prosecution.

England’s privatised water firms have been criticised for failing to invest in infrastructure while paying out billions in dividends to investors.

Last week shareholders in Britain’s biggest water company, Thames Water, said they had withheld £500m from the indebted utility after a standoff with the regulator, Ofwat, which is refusing their demands to increase customer bills by 40%.

In response, a bond issued by Thames Water’s parent company has now fallen to record lows. The £400m bond, which was issued by the water supplier’s parent company, Kemble and which matures in 2026, has lost more than 80% of its value over the past 12 months. It halved to only 12.7p after the shareholder update, before lifting slightly to 14.4p on Tuesday.

Kemble is expected to try to persuade its lenders to agree to a debt for equity swap.