Method of formulating a lateral flight trajectory for the rejoining by an aircraft (200) of a trajectory of a flight plan (910) comprising a plurality of waypoints (911, 912, 913), the aircraft (200) flying outside of the flight plan (910) and according to a divergent track with respect to the trajectory of the flight plan (210), characterized in that the method: determines a waypoint (913) of the flight plan for the rejoining, defined as the first waypoint (913) of the flight plan included in a capture zone defined by the flight plan trajectory situated downstream of the point of intersection between the straight line defined by an angle (α) with the perpendicular to the track of the aircraft and the trajectory of the flight plan, formulates the optimal lateral flight trajectory for a rejoining by the aircraft (200) at the determined waypoint. Advantageously, the flight trajectory formulation method allows the application of speed and/or altitude constraints to the aircraft (200) taking into account speed and/or altitude constraints imposed along the trajectory of the flight plan (910).