Five fire crews from across Lancashire were mobilised to the Bull and Butcher in the early hours of Monday morning to combat the blaze, which is believed to have been started deliberately.
The damage was so severe to the first floor of the former pub that contractors have been called to pull part of it down.
Firefighters used four jets and an aerial ladder platform to extinguish the fire.
Watch manager Gary Drinkwater said: “The fire was very severe and took up a lot of resources from across the region including a cherry picker which came from Preston.
“The fire appears to have been started deliberately on the ground floor which then spread upstairs and into the roof.”
The part of the building, which was the former Ashoka restaurant, was untouched by the fire.
Planning permission was passed last year to transform the pub into an Italian restaurant.
Fire crews were still damping down at the scene on Monday morning.