Crews forced their way inside the empty department store and brought the fire under control using hose reels.
They were stood down around 5am after using thermal imaging cameras to check for hotspots.
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The unit has remained empty since BHS fell into administration in 2016 and vacated the store.
“An investigation is now underway to determine the cause,” a spokesman for Lancashire Fire and Rescue Service told the Post.
At this stage, it has not said how serious the fire was or whether it is believed to be suspicious or not.
It is the latest fire to break out inside buildings standing empty in Preston city centre in recent weeks.
In May, the former Odeon cinema in Church Street had to be demolished after a devastating blaze gutted the historic building.
Just days earlier, the derelict St Joseph's Orphanage – abandoned 19 years ago – was seriously damaged when a suspicious fire tore through the site.
Both fires were deemed suspicious and three teenagers were arrested in connection with the Odeon cinema blaze.
The boys, two aged 14 and a third aged 16, were arrested on suspicion of ‘arson endangering life’.
A fire service spokesman said: “Four fire engines from Preston, Bamber Bridge and Penwortham attended a commercial building fire on Fishergate.
"Firefighters wore breathing apparatus and used one hose reel, two thermal imaging cameras, a saw and ventilation units to extinguish the fire.”