A one-piece or monolithic body flat plastic lid for carton packages with thin walls and membrane or a web breaking under pressure, with a semicircular skirt provided with rectangular shaped entrance housing and locked to a closing and opening pivoting tab, joined to the border of a pouring orifice by means of a rupture horizontal perimeter membrane and a vertical breaking or tearing by means of pressure exerted in the vertical or transversal direction to the surface of the tab, which is locked by means of a hinge line to a transverse wall making the rectangular channel.