A cutting tool has a cutting insert removably secured to a tool body by a fastening member. The cutting insert includes a clamping bore having at least two upper and at least two lower clamping portions intersecting a first and a second horizontal plane, respectively, the first and second horizontal planes being perpendicular to a central axis and spaced apart from each other, and the clamping bore being non-circular at cross-sections taken in the first and second horizontal planes. Upper and lower imaginary lines tangential to the at least two upper and the at least two lower clamping portions, respectively, form zero or acute first and second clamping angles with the central axis, the first and second clamping angles being different. Exactly two upper and exactly two lower clamping portions are in simultaneous clamping contact with an equal number of corresponding clamping zones on the fastening member.