The potential environmental risks due to gene flow from genetically modified (GM) crops into wild plant populations are currently of great concern. Of particular interest is gene flow from GM oilseed rape (Brassica napus) into its wild parental species B. rapa and B. oleracea, with which it forms spontaneous hybrids. The rate of hybridization is best determined empirically under a wide variety of field conditions. A procedure for quantifying hybrid formation using remote sensing over a large area of southern England is described. Satellite sensor images were used to identify localities where gene flow was most likely by searching for oilseed rape fields adjacent to regions where the parent plants occur, namely rivers and cliff-tops. These sites were visited and screened for hybrids. A single hybrid was observed in an area of over 16 000 km2. We use these data to suggest a strategy to minimise gene flow following commercial scale release of GM B. napus across Europe and propose a procedure that may allow post-release containment.