The chuck comprises a base body (1) having a central axis (12), an outer casing (2) axially fixed but rotatable about the central axis (12) outside the base body (1), the outer casing (2) having inner guides (15) and a plurality of jaws (3) movable along the inner guides (15), a fitting mechanism linking rotations of the outer casing (2) to movements of the jaws (3), and a locking device comprising a ratchet mechanism operated by an outer control sleeve (14). The ratchet mechanism comprises a toothed ring (5) fixed to the base body (1), a pawl fixed to a lever (10) pivotally mounted to the outer casing (2) about a pivot pin (9) parallel to the central axis (12), and an elastic element (28) urging the pawl (6) to engage 10 the toothed ring (5). The control sleeve (14) is rotatable and axially movable in cooperation with a cam (8).