Will Millard, who is also an ambassador for Keep Wales Tidy, shared a video on social media voicing his disappointment and said he was "incandescent".

His brother-in-law spotted the faeces in the River Edw in Powys and the incident has since been reported to Natural Resources Wales.

Millard told Sky News it was "incredibly selfish behaviour compounding the damage".

"The Edw is a hidden gem, just the most beautiful hill stream that is absolutely teeming with life, but it doesn't take much to cause huge irreversible damage," he said.

"Rivers are not toilets."

In the video posted on Twitter, Mr Millard said the discovery was "unbelievable".

"We're staying just in a little bunkhouse up there. Really, really beautiful, been coming here for years, especially just to come down here and do a bit of paddling in these pools with the kids," he said.

"My sister's here as well with her kids and all four of our kids were just in this little bit of river here, catching stone loach and enjoying the river when my brother-in-law just walked into this pool and said there's a load of human poo just off the bridge, and sure enough, all of these sandy looking patches are faeces and there's absolutely loads of it."

Mr Millard said it was "one of the worst things I think I've ever seen in a Welsh hill stream and it's appalling".

"All I can imagine is someone's come along here, probably with a chemical toilet or something having camped somewhere around here and just chucked it off the bridge," he added.

"I mean, terrible for our kids, they've all had to go back now and get washed up, but awful for the wildlife and the natural environment."

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Sky News has approached Natural Resources Wales for comment.