Staff at the Mind charity shop in Keynsham believe they are being targeted by someone with a grudge after a fifth arson attack on the premises since July.
The most recent incident last night was also the most serious, causing serious damage to the rear door of the shop.
Although flames penetrated the inside of the building, the firefighters were able to extinguish the blaze before it spread to neighbouring businesses or caused damage to stock.
Previous attacks have included two instances of the wheelie bin being set alight and another with a bag of clothes left as a donation. A settee and a wooden cabinet which had been delivered to the rear of the premises on Ashton Way were also piled up and made into a bonfire.
Assistant manager Sandra Parker was called out last night and had to arrange for the building to be secured well into the early hours of today The shop remains closed today.
She told The Week In that the attacks had upset staff working at the Mind shop, who are all volunteers.
She said: “We are asking that local residents and business owners can keep their eyes out for us and report any suspicious activity to the police.
“If anyone saw anything on Sunday evening around 11pm or has information regarding the other incidents, can they please call the police on 101.”