The fire which broke out in the Grand Hotel over the weekend was caused by an air conditioning and heating unit.
The specialist fire investigation team finished their investigation today after Saturday night's blaze at the five-star hotel.
About 400 guests had to be evacuated from the building at 8pm and were taken to nearby Hilton Metropole Hotel, where they were given hot drinks, sandwiches and blankets.
Ten fire engines and more than 50 firefighters from East and West Sussex went to the incident in King's Road, Brighton.
Group Commander Peter Cox, from East Sussex Fire an Rescue Service, said: "The most likely cause was a fault in an electrical appliance in the room.
"There has never been any suspicion. We have done a comprehensive investigation of the fire room and it left us with a fair amount of evidence that it was due to the air conditioning unit under one of the bedroom windows.
"There was a young couple staying in the room but they were not in the room at the time. It had been empty for about 30 minutes. The heater was left running, blowing hot air into the room."
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