A safety opening plug for a container, comprising: - a first part adapted to be integral with said container, defining a container outlet, - a second part pivotally mounted on the first part about a longitudinal axis of the plug, comprising a fixing ring and a cap secured to the fixing ring by a hinge, the first part comprising a rib defining a recess extending over part of the circumference of the first part and a groove extending from the recess towards the outlet, the cap comprising a lug configured to slide in the recess and in the groove, the groove being configured such that the second part of the plug can be manipulated relative to the first part between: - a closed position in which the lug is retained in the recess, with the cap closing the outlet orifice, - a release start position in which the lug is positioned in the recess facing the oblique groove, - an end-of-opening position in which the lug is positioned opposite the groove opposite the recess, said end-of-opening position being angularly offset from the beginning-of-opening position, and - an open position in which the cap frees the outlet orifice, wherein the lug and the rib have respective shapes adapted to enable closure of the plug by latching of the lug in the recess in any position other than the initial unlocking position, the plug being characterized in that a part of the rib arranged opposite the lug when the lug is in the unlocking start position forms a step sufficiently high to prevent the passage of the lug into the recess and thus prevents closure of the plug, and in which, when the lug is in any position other than the unlocking start position, the faces of the lug and of the rib which are opposite each other when the plug is in the open position have respective slopes conducive to sliding of said faces over each other, associated with elastic deformation of the cap, until insertion of the lug into the recess. - a first part adapted to be integral with said container, defining a container outlet, - a second part pivotally mounted on the first part about a longitudinal axis of the plug, comprising a fixing ring and a cap secured to the fixing ring by a hinge, the first part comprising a rib defining a recess extending over part of the circumference of the first part and a groove extending from the recess towards the outlet, the cap comprising a lug configured to slide in the recess and in the groove, the groove being configured such that the second part of the plug can be manipulated relative to the first part between: - a closed position in which the lug is retained in the recess, with the cap closing the outlet orifice, - a release start position in which the lug is positioned in the recess facing the oblique groove, - an end-of-opening position in which the lug is positioned opposite the groove opposite the recess, said end-of-opening position being angularly offset from the beginning-of-opening position, and - an open position in which the cap frees the outlet orifice, wherein the lug and the rib have respective shapes adapted to enable closure of the plug by latching of the lug in the recess in any position other than the initial unlocking position, the plug being characterized in that a part of the rib arranged opposite the lug when the lug is in the unlocking start position forms a step sufficiently high to prevent the passage of the lug into the recess and thus prevents closure of the plug, and in which, when the lug is in any position other than the unlocking start position, the faces of the lug and of the rib which are opposite each other when the plug is in the open position have respective slopes conducive to sliding of said faces over each other, associated with elastic deformation of the cap, until insertion of the lug into the recess. - a closed position in which the lug is retained in the recess, with the cap closing the outlet orifice, - a release start position in which the lug is positioned in the recess facing the oblique groove, - an end-of-opening position in which the lug is positioned opposite the groove opposite the recess, said end-of-opening position being angularly offset from the beginning-of-opening position, and - an open position in which the cap frees the outlet orifice, wherein the lug and the rib have respective shapes adapted to enable closure of the plug by latching of the lug in the recess in any position other than the initial unlocking position, the plug being characterized in that a part of the rib arranged opposite the lug when the lug is in the unlocking start position forms a step sufficiently high to prevent the passage of the lug into the recess and thus prevents closure of the plug, and in which, when the lug is in any position other than the unlocking start position, the faces of the lug and of the rib which are opposite each other when the plug is in the open position have respective slopes conducive to sliding of said faces over each other, associated with elastic deformation of the cap, until insertion of the lug into the recess.