A 200-year-old textiles mill could be converted into modern apartments if plans submitted to Stroud District Council are approved.
A public consultation is under way on the plans for Stanley Mill, a Grade I listed building in Stonehouse.
The current owners want to sell it as they cannot afford the upkeep.
But historian Anthony Burton described the building's interior as "magical" and said: "Once you split the building up, that's lost forever."
'Cute little balconies'
Plans for the mill, which closed 20 years ago, include up to 150 apartments and houses, a new factory building and parking.
Mr Burton, from the Stroudwater Textile Trust, said: "We're not opposed to development per se, but we are opposed to the way in which this is being done. This is an absolutely unique building.
"You may not even get an impression of that from looking at the outside - which looks like another mill building - but once you get inside you discover it's an absolutely magical place, with these wonderful iron colonnades on every floor.
"I cannot believe for example, on a Grade I listed building, that they're suggesting that they'll put cute little balconies on the outsides of windows."
Jill May, managing director of Stanley Mill, said: "You find yourself responsible for the final salary pension scheme of the employees and at the moment it's the only way forward for the site and for the business."