We extend the classical Plateau–Rayleigh instability criterion in the \(\mathbb {E}(\kappa ,\tau )\) spaces. We prove the existence of a positive number \(L_0>0\) such that if a truncated circular cylinder of radius \(\rho \) in \(\mathbb {E}(\kappa ,\tau )\) has length \(L>L_0\) , then it is unstable. This number \(L_0\) depends on \(\kappa \) , \(\tau \) and \(\rho \) . The value \(L_0\) is sharp under axially symmetric variations of the surface. We also extend this result for the partitioning problem in \(\mathbb {E}(\kappa ,\tau )\) .