The mega-eruption of Toba, Sumatra, occurred around 73 Ka ago, during the onset of a glaciation of the Late Quaternary. This coincidence combined with the unprecedented amount of sulphur released by this volcano has led to the hypothesis that Toba sulphate aerosols caused a transient surface cooling which may have contributed to a shift of the climate system. Because of the self limiting effect of gravitational sedimentation, the climatic impact of extremely large sulphur injections into the stratosphere are thought to be rather limited. Here we present model calculations combining microphysical and chemical feedbacks which show that the eruption could instead have led to the formation of a long-lasting volcanic aerosol layer. Although the concentrations of radiatively active species such as O3 or SO2 could also have been considerably perturbed, the resulting forcings should have only slightly moderated the aerosol cooling effect during the first few years following the eruption. According to our results, extremely high stratospheric sulphur loading could lead to a more prolonged effect on the climate than previously assumed.