The walls of what is thought to be one of medieval England's earliest and largest leprosy hospitals have been uncovered in Lincoln by Time Team archaeologists.

A three-day dig started on Friday on the South Common at the site of the Hospital of the Holy Innocents, which housed up to 20 people with leprosy around the late 11th to early 12th Centuries.

Time Team's Prof Carenza Lewis said they had found traces of square features where it is thought people were housed, which "haven't been excavated before, anywhere".

The dig marks the beginning of a three-year project backed by the National Lottery Heritage Fund.

The remains of the building were found last year during a Local Landscapes and Hidden Histories project.

More than 100 volunteers were involved in excavating test pits which uncovered what were thought to be the medieval boundary walls of the hospital.

Time Team carried out a geophysical survey of the area, like an X-ray of the ground, and volunteers have dug a 30m-long (98ft) trench to join up the two test pits where traces of the walls were found.

Lewis said a wall runs all the way down the trench and either side of it, according to the survey.

"We know from historical documents that there were up to 20 lepers housed here, and we know that they had little sorts of curtilages… which implies they're separate little buildings, little houses with a little yard, perhaps," she explained.

"What we've got all along this wall here are traces of little square features, about 5m (16ft) across, that are big enough to be a little building or a yard area, and we've got lots of them."

City of Lincoln Council said it believed the hospital remained in use for about five centuries.

The Hidden Histories Unearthed project, supported by £248,139 from the National Lottery Heritage Fund and led by Anglia Ruskin University, aims to uncover more about the people who lived and died at the hospital.

"You'd need to have resources to get in here," Lewis said.

"If you had enough money and could buy your way into a leper hospital, it's a bit like getting a place in a private care home or something like that."

The council said local residents would have opportunities to get involved in further archaeological excavations and activities as the project continues.

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