Workers looking for a place to rebury coffins in Bath Abbey have uncovered a complete medieval tiled floor buried two metres (6.5ft) down.
It is believed the tiles date from the late 13th or early 14th Century.
Bath Abbey is currently undergoing a £19m programme of refurbishment to stabilise its collapsing floor.
Cai Mason, senior project officer at Wessex Archaeology, described the find as the "most significant in the abbey's history".
It is estimated as many as 6,000 bodies have been buried below the building, which has left huge voids under the abbey's floor over time.
Some of the bodies have been found squashed flat and covered in bootprints from Victorian workmen who renovated the Abbey in the 19th Century.
"We came to dig a trench to rebury some of these coffins and came across a highly decorated tiled floor," said Cai.
"When we found this floor, it brought work to a halt while everyone came to gawp. We usually find fragments but it's very rare to find a whole floor.
"It's been buried here for about 500 years, covered in rubble from the earlier cathedral when the current building was built," he added.
The tiles will be preserved in situ, and will be documented for future generations by the archaeologists.