The man has already struck at Clock Face, Sutton, Whiston, Rainhill and Allerton in the last few weeks and, in the last raid, forced the safe to be opened before locking two staff members and a delivery man in a room.
The chain has 47 shops in the region. No-one was hurt in the incidents, but staff were left shaken and varying amounts of cash were stolen. Owner Richard Waterfield said: "We've no idea who this person is or why he is doing this there have been no dismissals or staff grievances that could be connected. We also trade in towns against Greggs and Greenhalgh's, which have similar banking procedures."
The chain has introduced new security measures, including reducing the float in its shops.