Firefighters battled a blaze at the Barnado's Donation Centre in Blackpool Road at 1am this morning.
The fire engulfed the exterior cladding of the building before spreading to Wyndsors World of Shoes next door.
Investigators believe the fire began in a waste bin outside the charity shop before spreading to the buildings. They are treating the fire as potential arson.
Four fire engines from Preston, Fulwood and Penwortham battled the blaze for an hour-and-a-half, using hose reels to bring it under control.
Lancashire Fire and Rescue Service reported the fire to police after suspecting that the fire was started deliberately.
A police spokesman said: "We were called at 1.23am this morning to reports of a fire on Blackpool Road, Ashton-on-Ribble.
"It was reported some waste bins were on fire next to the shops.
"The fire is being treated as suspicious and we are investigating.
"Please call 101 quoting Crime Ref: 04/11462/19 with any information."
Ian Armistead, station manager, said the building's cladding had protected the premises and prevented the fire from spreading.
He said: "I can confirm that the cladding/ panels fitted to the building did help prevent the fire spreading internally to the premises, allowing our crews to quickly extinguish the fire without it spreading any further than to the external panels.
"Obviously the premises was closed at the time of the fire and there isn't a sleeping risk, so there is a big difference in the cladding risk here than in the Grenfell Tower fire for example."
Staff at the Barnado's store confirmed that the store will remain open, but donations are not being accepted.
A Barnardo’s spokesperson said: “Damage has been caused to the front of the Barnardo’s charity shop on Blackpool Road, Ashton after a fire started outside in the early hours of Thursday, January 3.
"The shop has thankfully been able to reopen to customers today and we are supporting police with their enquiries into this matter.”
Wyndsors World of Shoes also confirmed that the store will open as normal.