The regulator described the incident, which took place in July last year near Pontithel, Powys, as having a highly detrimental effect on the river. It said its officers had been “appalled” by the damage caused.
However, in a statement issued last week it said that with “the deepest regret” it had ended its investigation as it had deemed there was “no realistic prospect of conviction against any company or individual”.
Guardian columnist and environmental campaigner George Monbiot described the news as “depressing but unsurprising”.
“It's a result of the massive funding cuts suffered by regulators throughout the UK. NRW had only one inspector covering a vast region, and he was on another call when this catastrophe was reported. Polluters are almost guaranteed to get away with it, however great the crime,” he tweeted.
One Powys county councillor tweeted that NRW was "not fit for purpose", while another said the collapse of the investigation was the “darkest hour in NRW’s history”.
Emily Durrant, a Green Party councillor in Powys, said she was "devastated" by the decision, adding that NRW was "not fit for purpose if it can't even enforce a major point source pollution event like this one".
William Powell, a Liberal Democrat councillor in Powys, tweeted that it was the "darkest hour in the history" of the NRW as "residents ... and all fair-minded people have waited patiently for justice".
Ann Weedy, operations manager for NRW in mid Wales, noted that the river Llynfi is a Site of Special of Scientific Interest, located in a Special Area of Conservation.
“The fact that such a significant number of aquatic animals died in the incident shows just how rich the wildlife was in this small tributary river,” she said.
“We have exhausted every lead and done everything we can to be able to prove what happened, but with the deepest regret, this investigation has now ended. We will consider reopening the investigation if compelling new evidence comes to light. If anyone has such evidence, we urge them to contact us,” she added.