The CEP (congruence extension property) or an algebra is characterized by means of modular pairs in its lattice of weak congruences. Hence the necessary and sufficent conditions are deduced under which all homomorphic images of an algebra having the CEP, satisfy the same property. This characterization is purely lattice theoretic, so two special cases are considered for which some sufficent conditions (modularity of congruence lattice, for example) are given.