A high sensitivity optical fibre temperature sensor is demonstrated based on a balloon-shaped bent single-mode (BSBS) fibre structure where the fibre retains its original protective polymer coating. The BSBS fibre structure can be simply realized by bending a coated straight single-mode fibre into the balloon shape using a section of silica capillary tube. By adjusting the bending radius of the balloon-shaped fibre section, a modal interferometer between the core mode and the coating mode can be effectively implemented at a suitable bending radius. Considering the intrinsically high thermo-optical coefficient and thermal expansion coefficient of the polymer coating, the BSBS fibre structure offers excellent temperature sensing performance. Experimental results show that the temperature sensitivity is as high as -2465 pm degrees C-1 with a resolution of 0.008 degrees C over the temperature range of 20.7 degrees C-31.7 degrees C. Based on its simple fabrication process, very low cost, and experimentally determined high sensitivity coupled with good repeatability, the temperature sensor described in this article could be a competitive candidate in many temperature sensing applications.