Bath House Barking, which operates the sauna, describes itself as a “holistic spa with a difference” with a range of massage treatments also on offer.
A spokeswoman for the Bath House said: "We still don't know what went wrong, we have to wait for the investigation. Luckily no one was in the spa so nobody was hurt.
"The Bath House will be closed for a few days so we can get rid of the smell, the Sauna has been burnt out but the treatment rooms are okay. We hope to be open later this week."
A spokesman for Barking and Dagenham Council said: "The fire is thought to have started in a sauna on the first floor of the building. There has been minimal damage and all other facilities in the building including the library, college and citizen advice bureau will be open tomorrow."
