The outlines of three buildings in Priory Park were detected by ground-penetrating radar last year and confirmed by a small trial trench, but extensive remains have now been found by local volunteers joining professional archaeologists in an excavation. The initial guess that the distinctive rounded end of one of the buildings could mean it was a bath house has been confirmed.

James Kenny, an archaeologist, said: “It’s almost unique to see Roman remains survive in this type of setting and to be so complete.”

The dig has uncovered the remains of the hot room and its hypocaust, the pillared basement that heated it. It would originally have been part of a suite of bathrooms attached to an opulent private house on the edge of the city, away from the noise and smells of the central markets area.

The remains have survived because in the densely built medieval city that grew up within the Roman walls, the site remained open land, which was eventually given to the city as a first world war memorial by the Duke of Richmond. Roman fragments have been found throughout the city, and the remains of a public bath house have been preserved within the Novium museum.

Kenny said: “The location definitely marks one of the most affluent parts of the Roman town, with these houses being the equivalent of a property worth millions of pounds in today’s society. Only someone who was incredibly wealthy could have owned a bath house like this and paid for it to be maintained. They would have either made their money out of commerce and trading, or were a member of the local aristocracy.

“We don’t know the date of the site yet, but it was probably third or fourth century AD. This was quite late in the history of Roman Chichester – but at the time when it was in its heyday.”

The remains will be buried again when the excavation is complete: more work is planned on one of the houses. Any surviving above-ground masonry was removed centuries ago for the Norman castle and monastic buildings.