The role of deep defects and their physical origin in InGaN/GaN LED are still widely investigated and debated, in particular their impact on the electrical and optical characteristics and on the reliability of the device. In this paper we evaluate the electrical and optical behavior of a single quantum well InGaN/GaN 420 nm LED during a medium-term accelerated ageing test. From these measurements, an increase in the sub-turn on current was observed: this process was ascribed to the stress-induced generation of defects in proximity of the active region and thus related to an increase of the correlated trap assisted tunneling. Subsequently we modelled the device electrical characteristics, evidencing the fundamental role of deep defects in determining the shape of the I-V curve in low forward bias. We reproduced with high accuracy the experimental current-voltage curves acquired during the ageing test, demonstrating that the increase in forward leakage can be entirely ascribed to the increase in concentration of a sole defect deep level located near midgap. These results provide a better understanding about the defect formation mechanism, that can be used to model and understand the behavior of the devices during ageing.