A historian has said there could be up to 122 executed criminals buried in unmarked graves under Gloucester prison.
The jail will close at the end of March and proposals for its future already include a hotel, flats and a museum.
But English Heritage said the site required archaeological investigation before any development takes place.
The Ministry of Justice (MoJ) said it was aware of 17 burials there between 1874 and 1939.
A MoJ spokesman said it was considering the future of these along with the future use of the site.
The prison, a Grade II*-listed building, was built over the levelled remains of Gloucester Castle.
Local historian Phil Moss said public executions took place at the gate lodge up until the middle of the 19th Century.
"Tradition was prisoners were always buried within the prison," he said.
Mr Moss added that there could also be Roman remains beneath the jail.
He said limestone set into the walls of the prison was believed to have come from “a Roman quay or something like that”.
But he added not all the bodies will have been buried there.
"According to the records when they were cut down from the gallows sometimes… the body was taken to Gloucester Infirmary and used for anatomical dissection," he added.