Context :

Aims : We undertake an optical and ultraviolet spectroscopic analysis of a sample of 20 Galactic B0 – B5 supergiants of luminosity classes Ia , Ib , Iab , and II .
Fundamental stellar parameters are obtained from optical diagnostics and a critical comparison of the model predictions to observed UV spectral features is made .

Methods : Fundamental parameters ( e.g. , T _ { eff } , \log L _ { * } , mass-loss rates and CNO abundances ) are derived for individual stars using CMFGEN , a nLTE , line-blanketed model atmosphere code .
The impact of these newly derived parameters on the Galactic B supergiant T _ { eff } scale , mass discrepancy , and wind-momentum luminosity relation is examined .

Results : The B supergiant temperature scale derived here shows a reduction of about 1 000 – 3 000 K compared to previous results using unblanketed codes .
Mass-loss rate estimates are in good agreement with predicted theoretical values , and all of the 20 B0 – B5 supergiants analysed show evidence of CNO processing .
A mass discrepancy still exists between spectroscopic and evolutionary masses , with the largest discrepancy occurring at \log ( L / L _ { \odot } ) \sim 5.4 .
The observed WLR values calculated for B0 – B0.7 supergiants are higher than predicted values , whereas the reverse is true for B1 – B5 supergiants .
This means that the discrepancy between observed and theoretical values can not be resolved by adopting clumped ( i.e. , lower ) mass-loss rates as for O stars .
The most surprising result is that , although CMFGEN succeeds in reproducing the optical stellar spectrum accurately , it fails to precisely reproduce key UV diagnostics , such as the N v and C iv P Cygni profiles .
This problem arises because the models are not ionised enough and fail to reproduce the full extent of the observed absorption trough of the P Cygni profiles .

Conclusions : Newly-derived fundamental parameters for early B supergiants are in good agreement with similar work in the field .
The most significant discovery , however , is the failure of CMFGEN to predict the correct ionisation fraction for some ions .
Such findings add further support to revising the current standard model of massive star winds , as our understanding of these winds is incomplete without a precise knowledge of the ionisation structure and distribution of clumping in the wind .