When groundwater leaks into the tunnel from damaged joints or cracks of the linings, fine particles could be pulled off from the soil matrix by seepage force and transported into the tunnel. Currently, very limited attention has been paid to the effect of the loss of fine particles induced by the water leakage, i.e. the internal erosion. In this study, the evolution of soil porosity, gradation, seepage flow, the induced ground movement and lining stress change due to tunnel leakage has been numerically investigated using a novel coupled hydromechanical approach formulated within the continuous porous medium framework. A critical state based constitutive model considering the influence of the fines content has been implemented for modelling the mechanical consequences of internal erosion. The numerical results show the spatial and temporal evolution of the eroded zone and the hydromechanical response of the tunnel and its surroundings. The results indicate that the commonly used pore pressure reduction-based method without considering internal erosion will underestimate the leakage induced lining stress change and ground movement. Moreover, the influences of three-dimensional condition are highlighted.