An advanced design of sandwich structures requires not only knowledge of global stress- and deformation behaviour, but also knowledge of local effects, such as load singularities and loss of stability caused by short wave wrinkling of one (bending load) or both (compressive load) sandwich skins.Based on the nonlinear theory for sandwich shells with seven kinematic degrees of freedom, introduced by and , an improved theory for plane sandwich shells with eight degrees of freedom is presented, taking into account the core warping, which enables a much better representation of the sandwich core behaviour. Because of consideration of quadratic core thickness, linear core shear strain, and longitudinal core deformation, prediction of wrinkling behaviour can be improved even for moderately thick cores with comparably thin skins. The kinematic quantities as well as the nonlinear differential equations and the simplified equations of first-order theory resulting from them are presented. Short numerical examples demonstrate the efficiency of the theory.