Regulator first stage for two-stage delivering assemblies, comprising a first chamber for a breathable high pressure gas, the chamber being connected or able to be connected through an inlet to a high pressure gas source; a second chamber for the breathable gas at an intermediate pressure, the intermediate-pressure gas chamber having an outlet for the intermediate pressure gas and being connected or able to be connected to a user of said intermediate pressure gas; a pressure reducing valve which connects said first chamber and said second chamber with each other and the valve comprising a valve seat having an opening for the communication between said first and said second chambers and a cut-off cooperating with said valve seat and displaceable from a closed position of said passage opening to an open position of said passage opening and vice versa, said cut-off being dynamically connected to a sensor member exposed to the pressure of the external environment of said two chambers, the member comprising a transmitting mechanism (321, 331) to transmit the mechanical stress exerted on said sensor member by the pressure of the external environment of the cut-off itself, wherein said sensor member sensing the external environment pressure, consists of at least one rigid element preferably and optionally of high mechanical strength (stainless steel or chromed brass) of movable wall which is sealingly slidable in a housing chamber, and which constitutes the interface wall of said housing chamber towards the external environment, and wherein said transmitting mechanism (321, 331) connects said movable wall with said cut-off, wherein the sensor element sensing the external environment pressure consists of two movable wall elements which are spaced apart from each other thanks to mutual connecting means in parallel with the sliding direction, one of said movable wall elements constituting the interface with the external environment and the other one constituting the interface between the housing chamber of said two movable wall elements towards the intermediate-pressure chamber, characterised in that both said two movable wall elements sealingly delimit, respectively towards the external environment and towards the intermediate-pressure chamber, an interposition chamber which is insulated from the external environment and from the intermediate-pressure chamber and which consists of a housing chamber portion having variable position and whose extent, in the sliding direction of the two movable wall elements, substantially corresponds to the distance between said two movable wall elements. a first chamber for a breathable high pressure gas, the chamber being connected or able to be connected through an inlet to a high pressure gas source; a second chamber for the breathable gas at an intermediate pressure, the intermediate-pressure gas chamber having an outlet for the intermediate pressure gas and being connected or able to be connected to a user of said intermediate pressure gas; a pressure reducing valve which connects said first chamber and said second chamber with each other and the valve comprising a valve seat having an opening for the communication between said first and said second chambers and a cut-off cooperating with said valve seat and displaceable from a closed position of said passage opening to an open position of said passage opening and vice versa, said cut-off being dynamically connected to a sensor member exposed to the pressure of the external environment of said two chambers, the member comprising a transmitting mechanism (321, 331) to transmit the mechanical stress exerted on said sensor member by the pressure of the external environment of the cut-off itself, wherein said sensor member sensing the external environment pressure, consists of at least one rigid element preferably and optionally of high mechanical strength (stainless steel or chromed brass) of movable wall which is sealingly slidable in a housing chamber, and which constitutes the interface wall of said housing chamber towards the external environment, and wherein said transmitting mechanism (321, 331) connects said movable wall with said cut-off, wherein the sensor element sensing the external environment pressure consists of two movable wall elements which are spaced apart from each other thanks to mutual connecting means in parallel with the sliding direction, one of said movable wall elements constituting the interface with the external environment and the other one constituting the interface between the housing chamber of said two movable wall elements towards the intermediate-pressure chamber, characterised in that both said two movable wall elements sealingly delimit, respectively towards the external environment and towards the intermediate-pressure chamber, an interposition chamber which is insulated from the external environment and from the intermediate-pressure chamber and which consists of a housing chamber portion having variable position and whose extent, in the sliding direction of the two movable wall elements, substantially corresponds to the distance between said two movable wall elements.