We assume that QCD can be described in a certain kinematical regime by an effective string theory. This hadronic string must couple to background chiral fields in a chirally invariant manner, taking into account the true chirally non‐invariant QCD vacuum. By requiring conformal symmetry of the string and the unitarity constraint on chiral fields we reconstruct the equations of motion for the latter ones. These provide a consistent background for the propagation of the string. By further requiring locality of the effective action we recover the Lagrangian of non‐linear sigma model of pion interactions. The prediction is totally unambiguous and parameter‐free. The estimated chiral structural constants of Gasser and Leutwyler fit very well the phenomenological values. © 2003 American Institute of Physics