[en] We point out that the world sheet swept by the confining string in the presence of dynamical quarks can belong to two different phases, depending on the number of charge species and the quark masses. When it lies in the normal phase (as opposed to the tearing one) the string breaking is invisible in the Wilson loop, while is manifest in operators composed of disjoint sources, as observed in many numerical experiments. We work out an explicit formula for the correlator of Polyakov loops at finite temperature, which is then compared with recent lattice data, both in the quenched case and in the presence of dynamical quarks. The analysis in the quenched case shows that the free bosonic string model describes accurately the data for distances larger than ∼ 0.75 fm. In the unquenched case we derive predictions of the dependence of the static potential on the temperature which are compatible with the lattice data