A pressure regulator includes a downstream body delimiting a low pressure chamber; an upstream body delimiting a high pressure chamber and carrying an expansion seat; a movable piston having an inner channel having an upper end forming a discharging seat and a lower end opening onto an intermediate chamber connected to a leakage circuit; a tubular valve rod passing through the inner channel of the piston and an inner channel of the upstream body, having an upper end forming a discharging valve and a lower end forming a regulating flap suitable for bearing on the expansion seat; a first biasing member compressed between the piston and the upstream body by biasing the piston towards the downstream body; and a second biasing member compressed between the piston and a plate secured to the valve rod, biasing the rod in the direction of pressing the discharging valve against the discharging seat.