Rebreather system comprising: - a mouthpiece; - a first breathing bag for exhaled breathing gas mixture in communication with said mouthpiece through a hose, particularly a corrugated hose; - a second breathing bag for the breathing gas mixture from which carbon dioxide has been removed, it also being in communication with the mouthpiece through a hose, particularly a corrugated hose; - a canister removing carbon dioxide from the exhaled breathing gas mixture interposed between said first and said second breathing bags; - a tank for a predetermined amount of a gas mixture replenishing the oxygen level in the breathing gas mixture where said replenishment gas mixture is stored at high pressure; - a tube supplying the replenishment gas mixture from the high pressure tank to the second breathing bag and in which tube there being provided a pressure controlled supply unit supplying a predetermined amount of replenishment gas mixture, which supply unit is of the servo-controlled type; - one or more sensors selected among the following sensors or combinations of the following sensors: pressure gauge, oxygen level sensor, carbon dioxide level sensor, depletion sensor for the canister removing carbon dioxide, filling level sensor for the tank with the replenishment gas mixture; - an electronic control unit that receives measurement signals from said sensors and that generates, as a function thereof, signals controlling said servo-controlled supply unit, wherein said supply unit comprises a servo-controlled supply valve of the normally open type, namely in absence of a control actuation signal it automatically and firmly takes the condition enabling the replenishment gas mixture to be supplied, characterized in that the electronic control unit is provided with a functional diagnostics automatic sub-system that in case of detection of one or more non-correct operating conditions of the electronic unit and/or of one or more peripherals connected to said electronic unit stops the said control actuation signal to the supply valve and irreversibly blocks the control signal to the supply valve that maintains it in the closed condition. - a mouthpiece; - a first breathing bag for exhaled breathing gas mixture in communication with said mouthpiece through a hose, particularly a corrugated hose; - a second breathing bag for the breathing gas mixture from which carbon dioxide has been removed, it also being in communication with the mouthpiece through a hose, particularly a corrugated hose; - a canister removing carbon dioxide from the exhaled breathing gas mixture interposed between said first and said second breathing bags; - a tank for a predetermined amount of a gas mixture replenishing the oxygen level in the breathing gas mixture where said replenishment gas mixture is stored at high pressure; - a tube supplying the replenishment gas mixture from the high pressure tank to the second breathing bag and in which tube there being provided a pressure controlled supply unit supplying a predetermined amount of replenishment gas mixture, which supply unit is of the servo-controlled type; - one or more sensors selected among the following sensors or combinations of the following sensors: pressure gauge, oxygen level sensor, carbon dioxide level sensor, depletion sensor for the canister removing carbon dioxide, filling level sensor for the tank with the replenishment gas mixture; - an electronic control unit that receives measurement signals from said sensors and that generates, as a function thereof, signals controlling said servo-controlled supply unit, wherein said supply unit comprises a servo-controlled supply valve of the normally open type, namely in absence of a control actuation signal it automatically and firmly takes the condition enabling the replenishment gas mixture to be supplied, characterized in that the electronic control unit is provided with a functional diagnostics automatic sub-system that in case of detection of one or more non-correct operating conditions of the electronic unit and/or of one or more peripherals connected to said electronic unit stops the said control actuation signal to the supply valve and irreversibly blocks the control signal to the supply valve that maintains it in the closed condition.