  We report a new first-order phase transition preceding capillary condensation
and corresponding to the discontinuous formation of a curved liquid meniscus.
Using a mean-field microscopic approach based on the density functional theory
we compute the complete phase diagram of a prototypical two-dimensional system
exhibiting capillary condensation, namely that of a fluid with long-ranged
dispersion intermolecular forces which is spatially confined by a substrate
forming a semi-infinite rectangular pore exerting long-ranged dispersion forces
on the fluid. In the T-mu plane the phase line of the new transition is
tangential to the capillary condensation line at the capillary wetting
temperature, Tcw. The surface phase behavior of the system maps to planar
wetting with the phase line of the new transition, termed capillary prewetting,
mapping to the planar prewetting line. If capillary condensation is approached
isothermally with T>Tcw, the meniscus forms at the capping wall and unbinds
continuously, making capillary condensation a second-order phenomenon. We
compute the corresponding critical exponent for the divergence of adsorption.
