Large gaps in basement membrane occur at sites of cell invasion and tissue remodelling in development and cancer. Though never followed directly in vivo, basement membrane dissolution or reduced synthesis have been postulated to create these gaps. Using landmark photobleaching and optical highlighting of laminin and type IV collagen, we find that a new mechanism, basement membrane sliding, underlies basement membrane gap enlargement during uterine-vulval attachment in Caenorhabditis elegans. Laser ablation and mutant analysis reveal that the invaginating vulval cells promote basement membrane movement. Further, an RNA interference and expression screen identifies the integrin INA-1/PAT-3 and VAB-19, homologue of the tumour suppressor Kank, as regulators of basement membrane opening. Both concentrate within vulval cells at the basement membrane gap boundary and halt expansion of the shifting basement membrane. Basement membrane sliding followed by targeted adhesion represents a new mechanism for creating precise basement membrane breaches that can be used by cells to break down compartment boundaries.