A balance spring includes a strip of polygonal section wound on itself into several coils. At least one coil of the balance spring includes a coil spacer device including at least four stop members mounted on the same face and protruding from the thickness of the at least one coil of the strip. Each of the at least four stop members is arranged to enter into contact with the opposite face of another coil of the strip preventing the other coil from touching the at least one coil in the event of a shock.