A nonlinear time-domain extension of the classical linear frequency-domain thin-shell approach is presented. The interface conditions are expressed in terms of the average magnetic flux density throughout the shell thickness and a number of higher order components. The method is elaborated in the frame of the magnetic vector potential formulation. The nonlinear system of algebraic equations is solved by means of the Newton-Raphson scheme. To validate the new formulation, we consider a magnetic plate placed above a double line carrying a sinusoidal current. Results are compared with those obtained with a fine finite-element model.