The ITER Fibre Optic Current Sensor (FOCS) is a dc coupled current measurement system that will provide back-up ITER plasma current measurements using a sensing fibre located on the outer surface of the vacuum vessel. To address the impact of a harsh tokamak environment on the sensor performance we installed a FOCS system on JET and performed current measurements in various machine operating scenarios for currents up to 3 MA. Me have observed a good agreement between the FOCS and the reference Continuous External Rogowski (CER) coils data. The linearity of the FOCS response was demonstrated. The FOCS has performance characteristics compatible with the ITER requirements and in contrast to the CER is non-sensitive to magnetic crosstalk effects.