Closure for a container , wherein said container has an open top side defined by an upper rim comprising an outwardly extending peripheral ledge surrounding said open top side, the closure comprising an annular base to be fixed to the upper rim of the container and defining an access opening to the container , and the closure further comprising a lid moveable relative to the annular base between a closed position, in which the lid covers the access opening , and an open position, in which the access opening is unblocked by the lid , the annular base comprising a peripheral skirt which has an integral non-removable end portion adapted to be arranged over the upper rim of the container , and furthermore comprising a peripheral locking rim integrally formed on said end portion of the peripheral skirt , said peripheral locking rim having a proximal end and a distal end , said proximal end of the peripheral locking rim being hingedly connected to the peripheral skirt , allowing the peripheral locking rim to be moved to a folded state, in which the peripheral locking rim is folded inwardly towards an inner surface of the end portion of the peripheral skirt , such that, when the locking rim is in the folded state, the distal end thereof can move beyond and engage under the peripheral ledge of the container to lock the annular base to the upper rim of the container , characterized in that the end portion of the skirt has on the inner surface an engagement surface portion with an inner diameter basically corresponding to the outer diameter of the peripheral ledge of the container and a widening surface portion having a gradually increasing inner diameter from the engagement surface portion towards an end of the peripheral skirt , wherein the inner diameter of the widening surface portion increases gradually according to a curved function, and wherein the widening surface portion adjoins the engagement surface portion in a flush manner. the closure comprising an annular base to be fixed to the upper rim of the container and defining an access opening to the container , and the closure further comprising a lid moveable relative to the annular base between a closed position, in which the lid covers the access opening , and an open position, in which the access opening is unblocked by the lid , the annular base comprising a peripheral skirt which has an integral non-removable end portion adapted to be arranged over the upper rim of the container , and furthermore comprising a peripheral locking rim integrally formed on said end portion of the peripheral skirt , said peripheral locking rim having a proximal end and a distal end , said proximal end of the peripheral locking rim being hingedly connected to the peripheral skirt , allowing the peripheral locking rim to be moved to a folded state, in which the peripheral locking rim is folded inwardly towards an inner surface of the end portion of the peripheral skirt , such that, when the locking rim is in the folded state, the distal end thereof can move beyond and engage under the peripheral ledge of the container to lock the annular base to the upper rim of the container , characterized in that the end portion of the skirt has on the inner surface an engagement surface portion with an inner diameter basically corresponding to the outer diameter of the peripheral ledge of the container and a widening surface portion having a gradually increasing inner diameter from the engagement surface portion towards an end of the peripheral skirt , wherein the inner diameter of the widening surface portion increases gradually according to a curved function, and wherein the widening surface portion adjoins the engagement surface portion in a flush manner. <EOS>