The process of unsteady two-dimensional boundary-layer separation at h
igh Reynolds number is considered. Solutions of the unsteady non-inter
active boundary-layer equations are known to develop a generic separat
ion singularity in regions where the pressure gradient is prescribed a
nd adverse. As the boundary layer starts to separate from the surface,
however, the external pressure distribution is altered through viscou
s-inviscid interaction just prior to the formation of the separation s
ingularity; hitherto this has been referred to as the first interactiv
e stage. A numerical solution of this stage is obtained here in Lagran
gian coordinates. The solution is shown to exhibit a high-frequency in
viscid instability resulting in an immediate finite-time breakdown of
this stage. The presence of the instability is confirmed through a lin
ear stability analysis. The implications for the theoretical descripti
on of unsteady boundary-layer separation are discussed, and it is sugg
ested that the onset of interaction may occur much sooner than previou
sly thought.