An ultraviolet (UV) optical fibre sensor with a narrow-band luminescent coating has been developed and tested in the presence of high-power UV light from an industrial scale microwave plasma UV lamp. A novel phosphor-epoxy mixture was used as a replacement cladding or coating for the fibre sensor. The combined absorption spectrum of the phosphor and the transmission spectrum of the epoxy result in the sensitization of the coating to a narrow band of wavelengths, centred around 254 nm. This paper describes the spectral characterization of the sensor in the laboratory and a comparison is made between it and a commercially available semiconductor-based UV sensor when illuminated by the lamp. The value of using the optical fibre sensor for this application is evident through this comparison, as the semiconductor sensor required extensive electromagnetic shielding prior to its use in the harsh environment of the microwave electric field. These sensors are likely to be used as on-line monitors for existing high-power UV sterilizing lamps for water supplies as European legislation increasingly requires the use of such monitors.