A highly birefringent silica microfiber was successfully developed by adiabatically tapering a low-birefringence flat fiber at a low temperature. Owing to the high fiber cross-sectional ellipticity and the large refractive index contrast between silica and the surrounding air $\left(\delta n=0.45\right)$ the flat microfiber provides an extremely high group birefringence ( $B=5.3\times {10}^{-3}$ at 1550 nm). The ability to obtain high birefringence in optical microfibers offers new possibilities for polarization discrimination and control within microfiber devices, such as high-resolution optical sensors, resonators, and fiber lasers.