A Bone anchoring device is provided comprising a bone anchoring element (1) having a shank (2) for anchoring in the bone and a head (3) a receiving part (5) for coupling a rod to the bone anchoring element, wherein the receiving part comprises a rod receiving portion (9) with a first end (9a) and a second end (9b) and a U-shaped recess (12) for receiving the rod, the recess extending from the first end in the direction of the second end thereby forming two free legs (12a, 12b) and a head receiving portion (17) at the side of the second end opposite to the first end for accommodating the head (3), the head receiving portion having a free end (17b) and being flexible so as to allow introduction and clamping of the head a locking ring (8, 8’) arranged around the head receiving portion wherein the locking ring can assume a locking position in which the head is locked by compression of the head receiving portion of the receiving part and wherein the locking ring (8, 8’) comprises an outer surface portion with an engagement structure (88, 88’) for engagement with a tool and a tool, and wherein a tool (50, 50’) is provided for releasing the locking of the head in a bone anchoring device, the tool comprising a tubular member (51, 51’) having a front end (52, 52’) and an engagement portion (54, 54a’, 54b’) at an inner wall of the front end for engaging the engagement structure (88, 88’) of the locking ring (8, 8’), a shaft (60, 60’) insertable into the tubular member (51, 51’) and movable relative to the tubular member in an axial direction, the shaft having a front end (61, 61’), wherein the front end (61, 61’) of the shaft is cnfigured to abut against a first portion (9a) of the receiving part when the engagement portion (54, 54a’, 54b’) of the front end (52, 52’) of the tubular member engages the engagement portions (88, 88’) of the locking ring, and wherein in this condition the tool is configured to move the shaft relative to the tubular member so as to move the locking
