Method of detecting a leak in a fuel system comprising a fuel tank and an orifice with a controlled section between the tank and the atmosphere, according to which a) the controlled section is set to a value Al at a time T1 and a pressure differential Δp1 between the inside of the tank and the atmosphere is measured at least after an interval of time ΔT from T1, for a constant fuel flow out of the tank; b) the controlled section is set to a value A2 at a time T2 and a pressure differential Δp2 between the inside of the tank and the atmosphere is measured at least after the same interval of time ΔT from T2, for the same constant fuel flow; c) a ratio of the pressure differentials Δp1 and Δp2 is computed and is compared to a reference pressure differential ratio ΔpL obtained with the same fuel system but comprising a calibrated leak.