This paper presents an analysis of the critical heat flux (CHF) of subcooled flow boiling based on the liquid sublayer dryout mechanism, i.e. the dryout of a thin liquid layer beneath an intermittent vapour blanket due to the coalescence of small bubbles. Starting from the same basic mechanism adopted in earlier models, a new model is derived for the analysis of the CHF in subcooled flow boiling under conditions of very high mass flux and liquid subcooling, typical of fusion reactors thermal hydraulic design. The model is characterized by the absence of empirical constants always present in earlier models. Predicted CHF values are compared with data of 1888 data points for water, showing a good agreement both in precision and in accuracy.