A jack (1), which is extensible, comprising: a first tubular body (5), a second tubular body (30) slidably associated with the first tubular body (5) and movable along a predetermined longitudinal direction (A), a third tubular body (95) slidably associated with the second tubular body (30) and movable along the longitudinal direction (A), a base (115) for support associated with an end (105) of the third tubular body (95) distal from the second tubular body (30), characterized in that it comprises an automatic safety device (145) provided with a lockbolt (150) movable between a first position, wherein it allows relative sliding between the third tubular body (95) and the second tubular body (30), and a second position wherein it prevents the third tubular body (95) from sliding relative to the second tubular body (30).