Closure for a container

A closure to be fixed to an upper rim of a container, the closure comprising:

• an annular base comprising a peripheral skirt which has an integral non-removable end portion adapted to be arranged over an upper rim of a 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,
• wherein the end portion of the skirt has on the inner surface an engagement surface portion with an inner diameter and a widening surface portion having a gradually increasing inner diameter from the engagement surface portion towards an end of the peripheral skirt;
• and
• a lid moveable relative to the annular base between a closed position, in which the lid covers an access opening, and an open position, in which the access opening is unblocked by the lid,
• 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.