This article is concerned with the stability analysis of quantised feedback control systems which contain the optimal dynamic quantisers recently proposed by the authors. First, it is shown that a sort of separation property of quantiser-controller design holds as far as the feedback system stability concerns. This allows us to design feedback controllers independently of the choice of quantisers. Then, based on this property, a necessary and sufficient condition is derived for the optimally quantised feedback system to be stable, which is described in terms of the poles/zeros of a linear feedback system to be quantised. Finally, we provide a class of suboptimal dynamic quantisers for which the resulting quantised control systems are guaranteed to be stable.