The chief executive of Natural Resources Wales said taking firms to court would not achieve "environmental betterment at that point".

Clare Pillman was answering questions in parliament as part of a Commons' Welsh Affairs Committee investigation into water quality in Wales.

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

Ms Pillman said the regulator was trying to constantly "push for environmental improvement", but insisted that prosecuting water companies did not improve the environment.

She referred to a recent case in North Wales as part of her evidence.

"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.

"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," she said.

"The courts in Wales have not imposed the sorts of levels of fines that we have seen in England."

However, the water quality of Wales's rivers was not "where it should be", according to Ms Pillman.

"This is neither a simple issue, nor one that has arisen overnight, and it has many sources, and we as the regulator have many roles in respect of coming to a position where we have a clear path towards vastly improved water quality in Wales," she added.

"Monitoring is an area where I think we can do more and I think technology is advancing and there are ways of monitoring now that actually can really help us going forward."

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David Black, chief executive of Ofwat, the regulator for England and Wales, said it was "important" it had the power to issue fines.

"But equally it's important that companies put right what's gone wrong," he added.