This is the first of a series of research papers featuring structure-property relationships in UK-grown natural cellulosic fibres, a renewable resource which may be extracted from the stems of plant fibres. They are composed of cellulose, hemicellulose, and lignin, together with other constituents, and, unlike seed hairs, have structures which make the plant stems strong and stiff. In the last few decades, fibres from agricultural crops have been replaced by synthetic fibres. In particular, this has resulted in under-utilisation of flax and hemp fibres, both of which may be grown in the UK. Although many studies have been carried out on bast fibres, and considerable information can be found in the literature, much of the information is of academic interest only, and the instances of verification through commercial use are rare. The driving force behind the current work was the EC strategy to improve self sufficiency in fibres, in order to reduce imports, and to find a substitute for jute and coir in industrial applications. The present study was mainly focused on flax fibres of different origins, particularly the straw waste product from linseed oil production. Until recently, this straw was burnt as a disposal route. The aim of the present investigation was to understand the effect of the chemical composition of different flax fibres on various physical properties, and, to compare them with those of other natural cellulosic fibres. It was concluded that seed flax, i.e. the fibre from straw left after harvesting linseed, has good potential for use in technical textiles.