An investigation is under way amid reports a large section of Offa's Dyke ancient earthwork has been damaged near Wrexham.
North Wales Police say they been made aware of possible damage to the protected ancient monument near Chirk.
The Welsh government's historic environment service, Cadw, is also investigating.
The Offa's Dyke Association said it understood the damage was caused on private land.
"I'm horrified," said Jim Saunders from the association.
"It is very sad. It is a 1,200-year-old monument - you can't put it back. People are nibbling away at the dyke all the time. The ancient monument isn't going to get any better."
The linear earthwork stretching 177 miles (285 km) is a protected ancient monument built in the 8th Century.
It is thought that King Offa of Mercia built it as as a boundary between his domain and Wales.
It is said to be traceable along 80 miles (130 km) from the Wye valley to Wrexham.
A spokesperson for Cadw said: "An investigation is currently under way to determine whether or not an offence has been committed."
North Wales Police said they were awaiting more information from Cadw.
The area attracts up to 15,000 visitors a week during the summer, many of them walkers from all over Europe.
Rob Dingle, national trail officer for the Offa's Dyke footpath, which runs close to the damaged section, said it was "one of the worst examples of direct damage to the monument that I've heard about", while local historian Mark Williams said the damage looked "beyond repair".
Terry Evans, who represents the area on Wrexham Maelor Council, said: "It's horrific that something like this should happen to a 1,200-year-old historical monument.
"They should be made to put it back together. I just can't believe that it's happened."