Fifteen fire engines and around 100 firefighters were called to a fire at a spa building adjoining a hotel in Richmond Hill in Richmond.
A leisure complex and pool building was destroyed by fire. Part of the third floor and roof of the adjoining hotel annexe was also damaged by fire.
Station Manager Mike Cotton, who is at the scene, said: "Firefighters worked hard in very challenging conditions to bring the fire under control. We expect to be at the scene working throughout the day today (May 1) to extinguish remaining deep seated pockets of fire.
"The fire was in an annexe building and had been producing a lot of smoke.
"Local road closures remain in place so we would advise drivers to avoid the area if possible and to check their journey before travelling."
Approximately 300 people left surrounding buildings as a precaution and hotel guests have been provided with alternative accommodation. There are no reports of any injuries.
The Brigade was called at 1824 and the fire was brought under control at 0136 (May 1). Firefighters remain at the scene throughout Wednesday. Fire crews from Richmond, Twickenham, Kingston, Heston and other surrounding fire stations attended the scene.
The cause of the fire is under investigation.