A pollution spill on a River Wye tributary may have caused a total wipeout of fish life, it has been claimed.
National Resources Wales is still investigating the source of the pollution on the Afon Llynfi near Brecon, Powys which an angling society say could take "ten years to recover".
NRW said it had identified 200 dead fish over a seven-metre stretch of the river after the alarm was raised on Friday evening.
However, Gwent Angling Society believe as much as 5km of river has been hit, with the loss of ‘tens of thousands’ of fish.
The society club vice chairman David Collins said: "I can confirm that it is absolutely catastrophic.
“NRW’s initial report was just the tip of a very large iceberg, it was just the first warning of what might be something very severe."
David had spent the weekend also collecting samples along the river.
The same stretch of river was devastated by a previous pollution incident in 2016 which killed stocks of trout, grayling and salmon.
The angling society said it had spent the past three years working alongside the Wye and Usk Foundation to restore the river to life.
Mr Collins continued: “We’ve put in a huge amount of effort, a huge amount of resources, and significant money.
“Literally only three days before the pollution, we were doing the very last bit of work to repair the damage from last winter’s floods."
NRW issued a warning on Sunday advising people not to swim in or near the affected waters.
“This investigation is a top priority for NRW and we are putting all the appropriate resources and expert staff in place to investigate it fully and to collect evidence," the statement said.
“The incident has had a devastating effect on the Afon Llynfi, a Site of Special Scientific Interest due to the local population of white-clawed crayfish.
"While we believe that the pollution has stopped entering the river, the investigation is ongoing, and we are committed to finding the source of the pollution.”