Ashbourne's swimming pool will be closed for the "foreseeable future" after a suspected arson attack on the leisure centre.
A fire which started in a bin at the rear of Ashbourne Leisure Centre off Clifton Road tore through the plant room of the public facility at around 11pm on Saturday.
Fire and rescue officers needed breathing apparatus to tackle the blaze.
Police have arrested a man on suspicion of arson.
The building has just undergone a £175,000 revamp after its owners Derbyshire Dales District Council used money from a Sport England Grant to overhaul the changing rooms.
The new "changing village" was being shown off to councillors on Friday - just over 24 hours before the fire started.
District councillor Tom Donnelly, who was at the sneak preview, said: "As the ward member for Ashbourne it's very disappointing when so much time and effort has gone in to making the building better for everyone in the town. I find it very upsetting that something like this should happen.
"I hope we'll be able to get the facilities up and running as soon as possible now but the smell of smoke in a building is a terrible thing and I'd imagine we may have to clean out the whole building."
While tackling the blaze, fire and rescue officers had to remove roof sheets to check for hot spots in the void above the centre and two of them entered an upstairs room, wearing breathing apparatus, to deal with a fire they discovered.
Staff are due to begin fully assessing the damage tomorrow but it is believed the swimming pool could be closed for several weeks due to damage to the plant room and equipment that runs the water system.
A spokesman for the district council said: "A fire at our Ashbourne Leisure Centre late on Saturday night means the pool is closed while a damage investigation takes place.
"Our dryside facilities, including fitness suite, are open as usual.
"At one stage the blaze - thought to have started outside the building - spread to the roof but, fortunately, the centre's new changing village, currently nearing completion, does not seem to have been affected."