A pressure reducing valve comprising: (a) a valve assembly defining a flow path from an inlet to an outlet; (b) a throttling arrangement forming part of said flow path, said throttling arrangement comprising a displaceable tube having a rim, said displaceable tube being displaceable relative to a closure surface of a plug between a fully-open position in which said rim is spaced from said closure surface so that said flow path is open, said flow path passing between said rim and said closure surface and through said displaceable tube, and a closed position in which said rim closes against said closure surface so that said flow path is substantially blocked; (c) a spring acting to displace said displaceable tube to said fully-open position; (d) a control chamber including at least one pressure-actuated piston surface integrated with and circumscribing said tube such that pressure within said control chamber acts to displace said displaceable tube against said spring towards said closed position; characterized in that the pressure reducing valve further comprises (e) a switchable hydraulic control circuit in fluid connection with said inlet, said outlet and said control chamber, said switchable hydraulic control circuit being configured to assume: (i) a first state in which a pressure within said control chamber (Pc) is equalized with an outlet pressure (P 2), such that a balance between the control chamber pressure and the force of said spring achieves outlet pressure regulation, and (ii) a second state in which the pressure within said control chamber (P C) is equalized with an inlet pressure (P 1), thereby overcoming the force of said spring to close the valve. (a) a valve assembly defining a flow path from an inlet to an outlet; (b) a throttling arrangement forming part of said flow path, said throttling arrangement comprising a displaceable tube having a rim, said displaceable tube being displaceable relative to a closure surface of a plug between a fully-open position in which said rim is spaced from said closure surface so that said flow path is open, said flow path passing between said rim and said closure surface and through said displaceable tube, and a closed position in which said rim closes against said closure surface so that said flow path is substantially blocked; (c) a spring acting to displace said displaceable tube to said fully-open position; (d) a control chamber including at least one pressure-actuated piston surface integrated with and circumscribing said tube such that pressure within said control chamber acts to displace said displaceable tube against said spring towards said closed position; characterized in that the pressure reducing valve further comprises (e) a switchable hydraulic control circuit in fluid connection with said inlet, said outlet and said control chamber, said switchable hydraulic control circuit being configured to assume: (i) a first state in which a pressure within said control chamber (Pc) is equalized with an outlet pressure (P 2), such that a balance between the control chamber pressure and the force of said spring achieves outlet pressure regulation, and (ii) a second state in which the pressure within said control chamber (P C) is equalized with an inlet pressure (P 1), thereby overcoming the force of said spring to close the valve. (i) a first state in which a pressure within said control chamber (Pc) is equalized with an outlet pressure (P 2), such that a balance between the control chamber pressure and the force of said spring achieves outlet pressure regulation, and (ii) a second state in which the pressure within said control chamber (P C) is equalized with an inlet pressure (P 1), thereby overcoming the force of said spring to close the valve.