The native UK bluebell Hyacinthoides non-scripta is considered to be at risk from hybridization with naturalised non-native bluebells. The non-natives, likely to be hybrid themselves (H. x massartiana) between H. non-scripta and H. hispanica, occur in relatively small numbers throughout the UK range of natives. Full interfertility between taxa has been repeatedly asserted in reporting spread of non-natives and predicting genetic erosion or assimilation of the native. However, there have been no published data to support suppositions that non-natives arose from in-situ hybridization, or that natural hybridization represents an ongoing threat to the native bluebell. Here we first investigated hybridization potential via reciprocal hand-crosses and observed overlap in flowering periods of native and non-native bluebells, finding that flowering was largely synchronous and that seed set and early seedling survival were equivalent for between-taxon crosses. We then established an experimental array allowing natural pollination to occur among flowering plants, and determined the paternity of offspring using microsatellite markers. We found that natives were more likely to produce seeds than non-natives, and that paternities approximated three native to one non-native, regardless of the identity of the maternal parent. Our results demonstrate that hybridization in natural populations and introgression between natives and non-natives are possible. However, lower reproductive success of non-natives coupled with the massive numerical advantage of natives represents a substantial constraint against ‘extinction-by-hybridization’ of H. non-scripta in the UK.