A precursor for security fibres, a method of producing the precursor, a method of producing security fibres from the precursor a security fibre for use in a

A security fibre comprises a transparent impermeable substrate; and a printed region 4 printed on only one side of the substrate printed with an ink invisible light and fluorescent in light outside the visible spectrum. The printed region is visible from each of the printed and unprinted sides of the substrate. There is also provided a precursor for a security fibre comprising an elongate, visible light transparent, ink impermeable web of indefinite length, on only one side of which is printed multiple regions 4 of ink. A method of manufacturing the precursor comprises providing a reel of transparent impermeable web (23, Fig.3A) of indefinite length; unreeling the web to move the web to a gravure printing stage; applying fluorescent inks to regions of the web, on only one side of the web, by means of the gravure printing roll (25, Fig.3A) to form the precursor; moving the printed web to a coiling stage, where the web is coiled into a reel (27, Fig.3A) of precursor. The regions may be divided into adjacent sub-regions 5, 6, 7, 8 each of which is a different ink which fluoresces a different colour when illuminated by ultraviolet (UV) light of 245-345 nm.