1. A method for controlling injection of a reducing agent upstream from a catalyst in an exhaust line from a combustion engine, the method comprising: calculating an accumulation actual value (A 1 ) representative of a current accumulation in the catalyst of a reducing substance forming part of or formed by the reducing agent based on information from a computation model, wherein the model takes into account expected reactions in the catalyst under prevailing operating conditions, and the model continuously determines the current state of the catalyst, calculating an accumulation setpoint value (A 2 ) based on an emission setpoint value (E 2 ) and information from the computation model, wherein the emission setpoint value (E 2 ) is representative of a desired content, in exhaust gases leaving the catalyst, of an exhaust gas substance which, as the exhaust gases pass through the catalyst, is at least partly removed from the exhaust gases by the action of the reducing substance or formed by the action of the reducing substance, and the accumulation setpoint value (A 2 ) is representative of the reducing substance accumulation required in the catalyst under prevailing operating conditions for substantially achieving the emission setpoint value (E 2 ), comparing the accumulation actual value (A 1 ) with the accumulation setpoint value (A 2 ), and controlling the injection of reducing agent in the exhaust line based on the comparison between the accumulation actual value (A 1 ) and the accumulation setpoint value (A 2 ).