A perfect fluid cosmological model with a non-vanishing cosmological constant is investigated. Scalar cosmological perturbations about a homogeneous and isotropic background are described within a previously developed gauge-invariant thermodynamical formalism. For the limiting case that the back-reaction of the fluid on the space-time geometry is negligible and the latter is determined only by the cosmological constant, modelling the energy density of the false vacuum in the inflationary period, the gauge invariantly defined adiabatic energy density perturbations of the fluid on «comoving» hypersurfaces are shown to grow exponentially. Metric perturbations on these hypersurfaces are exponentially unstable as well