Crews were called to the hotel, which dates back to 1819, around 2030 BST on Saturday.
Much of the hotel's top floors and roof were destroyed by the flames and there was extensive water damage throughout the rest of the building.
Firefighters tackling the fire were faced with 30-foot flames and at one point had to be taken out of the building.
Ken Probin, from Warwickshire Fire and Rescue Service, said: "With the reconstruction that's going on at the moment, the upper floors are very unsafe and with the fire taking hold through the roof space, it did lead to very unsafe underfoot and overhead conditions for the fire crews."
The Regent Hotel, which was used by wealthy industrialists and people who wanted to sample the spa waters in the 19th century, has been surrounded by controversy in recent years.
Plans put forward to demolish part of the historic building were challenged by the campaign group CLARA. Local residents are now concerned about the future of the landmark building.
One resident said: "My biggest concern has to be, they have permission to knock two-thirds of a grade two listed building down, and with what's happened last night, might they actually get permission to knock the whole thing down now?
Cynthia Bolton, from Cubbington, held her wedding reception at the Regent 40 years ago. She said: "Maybe something will be done about it now - there's been a lot of controversy about it but hopefully it'll improve matters now."
But the developers say the fire will not change their plans and the main items of history from the building are safely stored.
Crews were called to the hotel, which dates back to 1819, around 2030 BST on Saturday.
Much of the hotel's top floors and roof were destroyed by the flames and there was extensive water damage throughout the rest of the building.
Firefighters tackling the fire were faced with 30-foot flames and at one point had to be taken out of the building.
Ken Probin, from Warwickshire Fire and Rescue Service, said: "With the reconstruction that's going on at the moment, the upper floors are very unsafe and with the fire taking hold through the roof space, it did lead to very unsafe underfoot and overhead conditions for the fire crews."
The Regent Hotel, which was used by wealthy industrialists and people who wanted to sample the spa waters in the 19th century, has been surrounded by controversy in recent years.
Plans put forward to demolish part of the historic building were challenged by the campaign group CLARA. Local residents are now concerned about the future of the landmark building.
One resident said: "My biggest concern has to be, they have permission to knock two-thirds of a grade two listed building down, and with what's happened last night, might they actually get permission to knock the whole thing down now?
Cynthia Bolton, from Cubbington, held her wedding reception at the Regent 40 years ago. She said: "Maybe something will be done about it now - there's been a lot of controversy about it but hopefully it'll improve matters now."
But the developers say the fire will not change their plans and the main items of history from the building are safely stored.
