A hydrogen generating system and a method of in situ hydrogen generation controlled on demand capable of reacting an aluminum-free metal reagent selected from alkali metals, alkaline earth metals, alkali metal alloys and blends consisting of alkali metals, alkaline earth metal alloys and blends consisting of alkaline earth metals and metal alloys consisting of at least one alkali metal and at least one alkaline earth metal, with water to obtain hydrogen and a residual reaction product comprising metal hydroxide selected from alkali hydroxides and alkaline earth hydroxide; and separating hydrogen from the residual reaction product; liquefying the metal reagent by heating to obtain liquid metal reagent under vacuum conditions; injecting the liquid metal reagent into a reactor (3) by metal reagent injecting means (7,8) and simultaneously injecting, by water injection system (13, 14, 15), a stoichiometric amount of water with respect to the amount of the liquid metal reagent being injected into the reactor (3) such that a controlled metal reagent / water ratio is maintained in the reactor (3); transferring hydrogen and the residual reaction product from the reactor (3) to separation means (4, 18); separating hydrogen from the residual reaction product; transferring separated hydrogen to hydrogen receiving means (5) and transferring the residual reaction product to metal hydroxide receiving means (6), keeping the metal reagent injecting means (7,8) water injection system (13, 14, 15), the reactor (3), the separating means (4, 18) and the hydrogen receiving means (5) free of oxygen by selectively providing a vacuum in the system.