An anti-theft system for securing a removable battery to a power tool. The system employs a tamper-proof retainer to ensure that the retail display of a combined power tool and its associated battery can be displayed as a single unit without fear of theft of the battery. This may be achieved by the tamper-proof retainer securing a battery release latch against its release position hence securing the battery to the tool in a manner preventing the removal of the battery by a potential thief. The tamper-proof retainer either prevents removal of the battery from the power tool, as removal of the tamper-proof retainer is impossible, or, if it is destroyed by a thief, then it permanently secures the battery to the power tool.