Fan y Big, which stands at 2,531ft (717m), is set in the Brecon Beacons and is tall enough to be classed as a mountain.
But satellite measurements have revealed that the drop between Fan y Big and the next peak falls five feet (1.5 metres) short of what is required for mountain status.
The Brecon Beacons National Park, in South Wales, announced on Facebook: “It is with great sadness and a heavy heart that we must announce that through the emergence of new technological data we have lost one of our Beacons.
“Perhaps, sticking to Welsh folklore and myth, which we are accustomed to in the Brecon Beacons, it would be more apt to state that one of our mountains has turned into a hill.”
The national park adds that classification requirements state that a mountain must be at least 2,000ft tall (610m) and have a minimum drop between the summit and col, the lowest point on a mountain ridge between two peaks, of 98.4ft (30m).
As a result Fan y Big has been taken off the Hewitts list which records mountains in England, Wales or Ireland over 2,000ft.
It was downgraded after Myrddyn Phillips Welshpool, Wales, surveyed the hill and recorded the data using satellite technology.
Brecons National Park says that, despite its reclassification, “Fan y Big will continue to be a mountain to be climbed, a peak to be reached, a summit worth seeing”.
Their statement adds: “Let the satellites and figures show what they may, but underneath the starry night sky, the Fan y Big Beacon will continue to be a pinnacle at Wales’ best destination.”