Rare finds have prompted archaeologists to rewrite the history of an ancient north Pembrokeshire stone.

The Trefael Stone, a scheduled ancient monument in a Nevern field, was originally thought to be an ancient standing stone, but is actually the capstone of a 5,500-year-old tomb, according to new research from a Bristol University archaeologist.

Dr George Nash and colleagues' excavations at the site indicate that the 1.2m high stone once covered a small burial chamber, probably a portal dolmen, Wales' earliest Neolithic burial-ritual monument type.

The stone has multiple cupmarks, circular holes gouged into its surface associated with ritual burial activity in the Neolithic and Bronze Age. As the first archaeologists to fully investigate the site, Dr Nash and his colleagues Thomas Wellicome and Adam Stanford found a further 30 cupmarks of varying size and quality on the stone, along with an array of prehistoric artefacts that led the team to suggest that this site was more than just a standing stone.

During last year's excavation season the team unearthed sherds of pottery which appear to date from the late Neolithic; two perforated, water-worn beads similar to those found at the Early Mesolithic coastal settlement site at the Nab Head on the Pembrokeshire coast; and the remains of human bones. The archaeologists plan to conduct radiocarbon-dating and other tests on these remains when the required permissions have been granted to remove the bones.

Dr Nash said: "It's an important and exciting discovery. It's a once in a lifetime find.

"The excavation of this monument gives archaeologists a rare insight into the ritual-funerary activity of Britain's earliest farming communities. What is more significant is the survival of pottery and human bone from this period within such acidic soils."

He added that a burial site of this age is very rare as intense farming practices since the 17th century have destroyed many ancient sites. Further excavations are planned for September this year.