On 25 March 2019, a Boeing 737-800 making a non-precision daylight approach to Lanzarote with line training in progress began descending below the correct approach profile shortly after visual acquisition of the runway at 10nm when a much lower altitude than specified was maintained, EGPWS ‘CAUTION TERRAIN’ and ‘TERRAIN PULL UP’ activations eventually followed. With the terrain below clearly visible, the autopilot and autothrottle were disconnected and the aircraft flown level until the correct profile was regained after which the flight was then completed without further event. The Investigation made no findings which would fully explain the procedural non-compliance involved.