A process for obtaining a synthesis gas (GS) at a predetermined high pressure corresponding to the ammonia synthesis pressure, in which there are hydrogen and nitrogen in a 3/1 molar ratio, comprising the steps of feeding a continuous flow of natural gas to a primary reforming step (RP) with steam and to a subsequent secondary reforming step (RS) with excess air obtaining a gaseous flow (GF) comprises hydrogen, excess nitrogen with respect to said molar ratio, undesired substances such as impurities and inerts and subjects said gaseous flow to a purification step comprising cryogenic rectification in a separator unit (S) obtaining a continuous flow of synthesis gas (GS) comprising hydrogen and nitrogen in a 3/1 molar ratio, and to a subsequent compression step up to a pressure value required for ammonia synthesis.