Ammonia is one of the most widespread foraminiferal genera worldwide. Three phylotypes (Ammonia sp. T1, T2 and T6), commonly encountered in the northeast Atlantic, are usually associated with the morphospecies Ammonia tepida. The biogeographic distribution of these three types was previously investigated in coastal environments around Great Britain based on genetic assignations. A new method was recently developed to recognize these three phylotypes based on morphological criteria (i.e. pore size and suture elevation on spiral side), avoiding the need to use molecular analyses to identify them. The results presented here allow us to validate the consistency of the morphometric determination method but also to define more precisely the pore size variability of each of the three phylotypes, which is a main criterion for their recognition. Moreover, these results, combined with earlier molecular and morphological data, enable us to refine the biogeographic distribution previously established by genetic analyses alone. The biogeographical distribution pattern presented here supports the putatively invasive character of Ammonia sp. T6, by suggesting that this phylotype is currently spreading out over large areas and is supplanting autochthonous phylotypes (T1 and T2) along the coastlines of the British Isles and northern France. In fact, only the southwest coast of England and Ireland and the northwest coast of France have not been colonized by Ammonia sp. T6 yet. Our results also suggest that within the areas colonized by phylotype T6, T2 may find refuges in the inner parts of estuaries. We further suggest that the absence of Ammonia sp. T6 in the western part of the English Channel may be explained by the general surface current circulation pattern, which impedes further expansion. The high reliability of the determination method of phylotypes T1, T2 and T6 based on morphology also allows us to quickly generate large datasets for sub-recent and fossil material. This new method will make it possible to gain an understanding of the ecological differences between the three phylotypes and of the historical changes in their distribution patterns (for example due to changing anthropogenic factors). Finally, it will allow us to confirm or invalidate the putative invasive character of phylotype T6.