A propulsion system of a spacecraft comprises a main tank adapted to contain a volume of propellant and a pressurizing gas which applies pressure to the propellant. The main tank comprises a membrane delimiting an upper volume to contain the pressurizing gas and an inferior volume to contain the propellant, the gas applying the pressure to the propellant by means of the membrane. A pressurization circuit is connected directly to the main tank. The propulsion system further comprises an auxiliary tank adapted to contain pressurizing gas. The auxiliary tank is connected directly to the main tank by means of the pressurization circuit. When in operation, gas contained in the auxiliary tank expands continuously with the gas contained in the upper volume of the main tank, the pressures prevailing in the upper volume of the main tank, in the pressurization circuit and in the auxiliary tank being identical. The auxiliary tank is dimensioned so that the maximal volume of propellant of the main tank is greater than the volume of propellant which the main tank can contain without an auxiliary tank.