We present a new mechanistic, phase field-based formulation for predicting hydrogen embrittlement. The multi-physics model developed incorporates, for the first time, a Taylor-based dislocation model to resolve the mechanics of crack tip deformation. This enables capturing the role of dislocation hardening mechanisms in elevating the tensile stress, hydrogen concentration and dislocation trap density within tens of microns ahead of the crack tip. The constitutive strain gradient plasticity model employed is coupled to a phase field formulation, to simulate the fracture process, and to a multi-trap hydrogen transport model. The analysis of stationary and propagating cracks reveals that the modelling framework presented is capable of adequately capturing the sensitivity to the hydrogen concentration, the loading rate, the material strength and the plastic length scale. In addition, model predictions are compared to experimental data of notch tensile strength versus hydrogen content on a high-strength steel; a very good agreement is attained. We define and implement both atomistic-based and phenomenological hydrogen degradation laws and discuss similarities, differences and implications for the development of parameter-free hydrogen embrittlement models.