An analysis is performed of the influence of a catalytic surface reaction on the gas phase ignition of a premixed combustible gas flowing past a hot catalytic flat plate. It is assumed that the catalytic surface reaction is infinitely fast so that chemical equilibrium is reached along the whole length of the plate surface. In the gas phase a finite-rate chemical reaction with a large activation energy is considered. The analysis makes use of the boundary layer approximation to describe the gas flow and of first order matched asymptotic expansions to define ignition. Explicit expressions are derived for the critical gas phase Damköhler number for ignition and for the distance from the plate leading edge to the onset of ignition. The ignition distances for a catalytic and a non-catalytic plate are compared. Longer ignition distances for the former case are predicted. The results of the analysis agree qualitatively with previous experimental observations.