The structure and stability of large scale electromagnetic perturbations in a slab current layer with a sheared magnetic field is analytically investigated in the paper. In addition to the growth of the 'usual' quasisymmetrical tearing perturbations such a layer can be subjected to the development of large wavelength 'special' modes with high growth rates. The structure of the 'special' modes is analogous to the structure of the cylindrical modes with azymuthal wave number m=1. The instability saturation mechanism related with the coupling of tearing perturbations with ion sound waves appears to be effective only for usual quasisymmetrical tearing perturbations. The destruction of magnetic surfaces related with the growth of magnetic island in the central parts of the layer smoothly proceeds to their rippling at the peripheral magnetic surfaces. This corresponds to the transition in the longwavelength limit from the tearing to the kink modes of the instability.