Both Nelson fire engines are attending a fire at Pendle Wavelengths in Leeds Road, Nelson.
Police are also at the scene and early reports suggest that the fire, which broke out shortly after 10am, started in the sauna and spread throughout the building.
Traffic in Nelson is reported to be at a standstill.
A customer notice has been tweeted by Pendle Leisure Trust which reads: “Due to an incident at Pendle Wavelengths the facility is closed for the rest of today.
“There are no casualties and all occupants have been evacuated safely to the Community Centre on Leeds Road.
“We would like to reassure everybody that this incident is now under control and we will keep you updated with further information as and when it becomes available.”