A set of panels includes first and second panels and a mechanical locking device for assembly 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 strip edge, and first and second locking strip surfaces respectively extending in directions substantially corresponding to those of the first and second panel surfaces. A locking element of the locking strip cooperates with a locking groove at the 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. A flexing groove extends from a first locking strip surface/first edge transition and into the first panel at an angle α from the first panel surface.