This paper presents a large flax fibres varieties intercomparison with the aim of studying the dependence of the mechanical properties (tensile strength, ultimate strain and Young's modulus) on the variety. Thus, a Student's test analysis permits to dissociate the effects of the statistical variations within a large samples population and the effects of the species on the fibres mechanical properties. Results evidenced no significant influence of the variety on the fibres mechanical properties. Experiments show that this can be explained by a similar biochemical composition whatever the species considered, in particular the proportion of cellulose which is the reinforcing phase of the flax fibre. In addition, the cellulose crystallinity rate is the same for all the varieties. Weibull statistical analysis permits to highlight that the flax fibre is as reliable as some synthetic fibre, opening the way to its use as reinforcement in structural composite materials. Furthermore, this analysis evidenced that only one kind of defects involves the fibre breaking. Thus, X-ray tomography permits to experimentally define this type of defects which are cavities, of about 10 m long, constituting the preferential zone of crack initiation leading to the fibre breaking. Therefore, the dispersion of the fibre mechanical properties seems to be due to the number and size of the defects located in the cell walls and also to the usual determination method of the fibre diameter. Finally, it appears that the choice of the cropped variety must be mainly determined by the pedoclimatic conditions and the yields rather than by the expected mechanical properties. The improvement of the cultivation and scutching processes could be the key of the increase of the fibres mechanical properties by limiting the creation of structural defects.