A set of panels includes first and second panels, and a mechanical locking device for locking by vertical relative displacement of the panels. A locking strip extends from a first edge of the first panel in a direction parallel to first and second panel surfaces of the first panel. The locking strip includes a locking element configured to cooperate with a locking groove at a second edge of the second panel for locking in a direction parallel to the first panel surface. Opposite edges respectively include cooperating tongue and tongue groove for vertical locking. The locking strip includes a flexing groove extending from the second locking strip surface into the locking strip, to increase a flexibility of the locking strip during the assembly.