A large body of work has been devoted to the accurate detection and simulation of wave breaking in coastal areas. It is a key process for a wide range of engineering activities and environmental issues. This has motivated the development of a variety of breaking onset criteria, such as kinematic criteria based on a maximum value (usually unity) of the ratio u c /c, of the horizontal particle velocity at the wave crest u c to its phase velocity c, both taken in the direction of wave propagation. Here, we investigate numerically the validity of this criterion in capturing breaking onset for solitary and quasi-regular two-dimensional shallow water waves using the Fully Nonlinear Potential Flow (FNPF) model by Grilli and Subramanya (1996). With this model, the propagation up to overturning of solitary waves over plane slopes, and solitary and quasiregular waves over a submerged bar, both initially specified as numerically exact FNPF waves, is simulated. In all cases, waves break as spilling or plunging breakers, initiated by the formation of a breaker jet near the wave crest. Results show that the location of the maximum fluid velocity u m on the free surface closely coincides with the location where the overturning jet is initiated. Based on this, a new breaking onset criterion is proposed as u m /c 1, which is shown to be more universal for accurately detecting wave breaking initiation than existing criteria based on the crest velocity.