The article considers the problem of stability of the inner sealing shell (liner) of a composite pressure cylinder, considered as an infinitely long isotropic cylindrical shell, located in an absolutely rigid environment, simulating the composite layer of the balloon and compressing the shell so that it can lose stability. Using the equations of the nonlinear theory of cylindrical shells, an exact solution is obtained that determines the critical pressure or the limiting value of the subcritical deformation of the shell. It has been established that the critical pressure and deformation depend on the connection between the inner isotropic and outer composite shells. Two limiting cases are studied – shells rigidly connected to each other, and shells unilaterally connected only in the radial direction in the absence of friction. Some possible intermediate variants are considered – shells connected on a part of the section contour. The result obtained earlier is refined, based on the assumption that the form of buckling of the inner shell is described by nonlinear equations of the theory of shallow shells. The obtained solution is compared with the published experimental results.