A fire at a Matalan store in Grimsby is being treated as suspicious.
The fire broke out at 1600 BST on Sunday, while staff were still in the building, on the Alexander Retail Park. The workers escaped uninjured.
Humberside Fire and Rescue Service said the fire caused "significant" damage to stock but crews managed to prevent major damage to the building.
A spokesman said the cause of the fire was being investigated but the timing was suspicious.
Richard Hannigan, deputy chief fire officer, told BBC News: "The reality is that most fires in buildings like this do occur during the night time hours and if they're accidental due to electrical faults that's when they are likely to happen and become quite severe.
"This one developed as a severe fire while there were still people in the building and that is unusual so we will be looking at it in a particularly careful manner."