Prosecuting water companies does not improve the environment, the CEO of Natural Resources Wales (NRW) has said.

NRW has not made any prosecutions anywhere in Wales over illegal sewage dumping for the last five years, according to a recent BBC report, despite data showing that Dwr Cymru (Welsh Water) has been doing so for years.

Professor Peter Hammond, an environmental investigator from the group Windrush Against Sewage Pollution, told the Welsh Affairs Committee that data from one sewage treatment plant in Cardigan seemed to show that every spill there for the last six years has been illegal.

When asked by MPs why NRW is not prosecuting water companies for illegal sewage dumping, the regulator’s CEO, Clare Pillman, said: “I think that going to court is in a way, the point at which regulation has failed.

“Likely it has a demonstration effect but what we are looking for is for compliance.

“At the point at which you’re prosecuting for the failure, you are not getting environmental betterment. Whereas what we’re trying to do all the time is push for environmental improvement.

“At the point at which you’re in court, yes, a fine can be imposed and that can work, but a recent case in North Wales; yes, it went through court; yes, there was a fine levied; did it solve the environmental problem? No.”

Professor Hammond said Welsh Water is more transparent with its data than other water companies in England which have refused to give him any data on untreated sewage spills for the last two years because they are subject to an Environment Agency investigation.

He told MPs: “Severn Trent, United Utilities and South West Water just refuse to give data. And it’s in a sense unfair on Dwr Cymru.

“They’re providing the data so any illegality is being exposed, these other companies are getting away with it.”

He added that water companies should also measure the volume of sewage discharged and not just the duration of time or overall number, as some water companies have been known to tank sewage from one site to another that is already discharging in order to reduce the number of reported spills.

Peter Perry, CEO of Welsh Water, said: “Of course, it’s challenging when our performance is scrutinised at this level, but that’s not unusual for us, that’s what our board does to us, and the fact that it’s coming into the public domain is something we would welcome.

“We’ve got nothing to hide and we’re very open to making sure that people that pay our charges in Wales: politicians, activist groups; we’re accountable to them.”

Dr Laura Foster, head of clean Seas at the Marine Conservation Society, said: “Prosecution is retrospective – it will not reverse the damage to the environment, wildlife, people and local businesses that results from sewage discharges.

“We need to see a proactive approach, which requires a properly funded regulator and, where necessary, (an) independent assessor, that has the capacity to monitor and fully enforce the law.

“The threat and impact of prosecution should be strong enough to act as a deterrent. However, repeated disregard for the law shows us that this is not happening.”

Severn Trent, United Utilities and South West Water have been contacted for comment.