Quick connection system comprising a female end piece provided with a U-shaped locking clip and a male end piece intended to come to nest in the female end piece, the wall of the female end piece having opposite slots through which the legs (3', 3") of the U protrude into the passage of said female end piece and the wall of the male end piece having an outer groove designed to receive, with immobilisation, said legs (3', 3") of the clip in a final nesting state of the male end piece in the female end piece, locked by said clip, said legs (3', 3") being extended by free segments (5, 5') and separated elastically by the male end piece during its insertion into the female end piece until the clip is received in the groove in the final nested state of the two end pieces (2 and 4), the clip then being substantially in a rest configuration, with the legs (3', 3") of the U not forced apart, the quick connection system further comprising a piece forming a preloading element for the clip and comprising housing sites (7, 7', 7") that are able and intended to cooperate by engagement with the two free segments (5, 5') in a first configuration corresponding to the rest configuration of the clip and in at least a second preloaded configuration of the clip in which the legs (3', 3") of the U are further apart than in the rest configuration but less far apart than during the passage of the maximum outer section of the male end piece during its insertion into the female end piece, which quick connection system is characterized in that a free segment (5, 5') respectively extends each leg (3', 3") of the U in the direction of the other leg (3", 3'), substantially closing the U by cooperation, in that the piece forming a preloading element for the clip is a removable added piece, and in that the housing sites (7, 7', 7") are arranged on said added preloading piece such that during the insertion of the male end piece, and before the final nested state is reached, the clip preloaded by said piece is released automatically from its preloaded configuration and can return to its rest configuration. the quick connection system further comprising a piece forming a preloading element for the clip and comprising housing sites (7, 7', 7") that are able and intended to cooperate by engagement with the two free segments (5, 5') in a first configuration corresponding to the rest configuration of the clip and in at least a second preloaded configuration of the clip in which the legs (3', 3") of the U are further apart than in the rest configuration but less far apart than during the passage of the maximum outer section of the male end piece during its insertion into the female end piece, which quick connection system is characterized in that a free segment (5, 5') respectively extends each leg (3', 3") of the U in the direction of the other leg (3", 3'), substantially closing the U by cooperation, in that the piece forming a preloading element for the clip is a removable added piece, and in that the housing sites (7, 7', 7") are arranged on said added preloading piece such that during the insertion of the male end piece, and before the final nested state is reached, the clip preloaded by said piece is released automatically from its preloaded configuration and can return to its rest configuration.