The aim of this study was to develop taper functions for plantation grown Sitka spruce (Picea sitchensis (Bong.) Carr.) and Scots pine (Pinus sylvestris L.) in Northern Britain. Data were collected from stands in Scotland and Northern England and several taper functions were tested. After model fitting and validation, three variable-exponent taper functions gave the best results and were refitted using non-linear mixed-effects techniques. The use of the mixed-effects modelling approach improved the precision of the taper functions by between 45 and 63 per cent. The taper functions which gave the best results based on fit and validation statistics, model simplicity and overall model behaviour are recommended as stem profile models for Scots pine and Sitka spruce growing in Northern Britain. The models obtained are useful in predicting diameters at any height along the tree bole and can be used jointly with diameter distribution models and height diameter models to define product assortments. They can also serve as link functions between tree growth and timber quality models and for biomass quantification in carbon accounting.