Human poo, food covered in maggots and a tent "full of kit" were left behind at a south Wales national park over the bank holiday weekend.
Bannau Brycheiniog National Park said there was "no justification" for fly-tipping after noticing a "change in attitude" among visitors to its beauty spots, leading to an increase in litter dumping.
"Unfortunately, across this summer, it's not been unusual," the national park authority said.
It described the incident, near Llyn y Fan Fawr in the west of the park, as "one of the worst" it had seen, following on from long-running wildfires in the area.
In a plea issued on social media this week, the park authority at Bannau Brycheiniog, also known as the Brecon Beacons, said: "This is not wild camping. This is fly-tipping and littering.
"One of our wardens spent their bank holiday weekend in a less-than-ideal way, clearing up a tent full of kit with chairs, a BBQ and two bags of maggot-infested food, and also found human excrement on site.
"When out enjoying nature, respect your surroundings. Leave no trace and take everything back home with you. There is no justification for this behaviour."
Bronwyn Larry, head of communications for the park authority, said this was not the first instance of wardens discovering "abandoned 'wild camps'".
"It does seem to be increasing," she said.
"People come to the area to enjoy the beautiful scenery, the beautiful views, but now we are being faced with litter and it's disturbing that beautiful view."
She said the authority found the shift towards more fly-tipping "baffling" and said they were struggling to "figure out how to talk to people" about the issue.
"It's not just here, for our park, it does seem to be across the national parks. We are constantly putting out messaging around The Countryside Code, external, and the algorithm that we follow... that's what I see, for me it's second nature to take back what you go with."
Social media has previously been suggested as a reason for a rise in the issue, with tourists visiting the "Instagrammable" national park urged to research and respect the area.
But Lally said that, while "many influencers visit the area", "generally their messaging is around visiting responsibly" because "they want it to be nice too".
"So it's really difficult to understand where this change of attitude is coming from," she said.
Lally said the weekend's fly-tipping was "one of the worst examples that we have come across", but it was not unusual to find discarded BBQs, beer cans and rotting food.
"Logic says it's lighter to carry back once you've eaten and drank it all than it is to carry it out there," said Lally.
"Come and have a nice time, but leave no trace."
She said the phrase "leave no trace", cited in the code, had been associated with beauty spots "for so long" but was "just as relevant today".
The Countryside Code also states that "dropping litter and dumping rubbish are criminal offences".
"Our wardens have gone from supporting firefighters with the wildfires and, the first rainy weekend, they're off finding abandoned wild camps. They have had quite the summer," said Lally.
"It has a knock-on effect on wildlife. There's still a fire risk. Pollution can contaminate water. It affects everything, not just it looking nice."
Steffan Edwards is a warden at Bannau Brycheiniog National Park and cleaned up the mess.
He told BBC Radio Wales Breakfast: "We've got plenty of things to get on with within the national park as warden's."
He said it was a "shame to see all that rubbish discarded on the hill" but added warden's like Edwards discover abandoned items "all the time", which they have to deal with.
"This isn't new to us and it's not new to other national parks and other beauty sites across Wales and the UK. But what we are finding is it's an increasing problem over the last couple of years.
"People are finding out about these nice places and taking literally everything up with them, which you would take to a campsite and just leaving it there."
Policy and Advocacy Manager at Wildlife Trust Wales, Joe Wilkins, said the discovery of the litter has come at a time where plants and animals "can't sustain another threat", following a summer of extreme heat.
Wilkins said it is "so important that people have access to these spaces" and the ability to enjoy them, adding it is "unfair" for people to "absolutely ruin that experience for others".
