We report the first demonstration of extreme ultraviolet (XUV) supercontinuum generation in the plateau region of the high-order harmonic spectrum indicative of a single attosecond pulse. It was accomplished by combining the generation of sub-10 fs laser pulses with the polarization gating of high harmonic generation. When an 8 fs pulse centred at 750 nm was split and delayed with a quartz plate and then recombined with a quarter waveplate, a laser pulse was created whose polarization varied rapidly from circular to linear and back to circular. The near-linearly polarized portion of the resultant pulse was only 1.3 fs long, which was much shorter than the laser pulse duration. Since the high-order harmonic generation process is susceptible to the ellipticity of the driving field, the pulse with a time-dependent ellipticity behaved like a half-cycle linearly polarized pulse for generating XUV radiation. By exciting argon gas with the pulse, a supercontinuum that covered 25-45 nm was produced, which corresponds to an estimated single 200 as pulse.