National parks authorities are clamping down on a race which has become so popular it is causing problems on Wales' highest mountain.
Snowdon's 1,085m is usually the last peak for most entrants
Every year thousands of the fittest hill walkers arrive at the foot of Snowdon as part of the Three Peaks Challenge, where they have 24 hours to climb the tallest spots in Wales, England and Scotland.
But it attracts so many contestants to north Wales, Scafell Pike in the Lake District and Ben Nevis that new rules have been drawn up.
It asks that the number of participants is limited, that vehicles are parked away from gateways and participants urged not to arrive, start or finish between midnight and 0500 GMT.
Contestants pack themselves on to the mountains, often at night, to complete the 500-mile route within 24 hours.
Park managers and local people living near Scafell say they have no facilities to handle the thousands of walkers who arrive en masse.
Last year 29,000 took part in the Three Peaks Challenge in June.
Guy Newbold, who runs a tourist website in the Lake District , said: "We can have 600 or 700 people turn up in the middle of the night.
"There's a problem with litter, and people are often tired after driving from Scotland or Wales, which poses a safety risk."
The Three Peaks website also offers advice to those hoping to compete in the challenge,
It states that the National Three Peaks Challenge has put a great amount of pressure on landscape and paths, car parks and services, local communities and even mountain rescue teams.
It advises challengers to ensure that they have a good standard of fitness and use equipment and clothing for mountain conditions, and have a good general understanding of mountain navigation.
The National Trust has also warned about the amount of pollution, litter and erosion caused by too many people.