An enhanced process for the preparation of urea from ammonia and carbon dioxide, at high temperature and pressure, with the formation of ammonium carbamate as intermediate, comprising the following steps in the synthesis section: (i) reacting ammonia and carbon dioxide at an overall pressure ranging from 12 to 20 MPa, with a molar ratio NH 3 /CO 2, as such or in the form of ammonium carbamate, ranging from 2.1 to 6, preferably from 2.8 to 4.5, in a reaction step carried out in a suitable vertical reactor R fed with at least one ammonia stream and at least one stream of fresh carbon dioxide containing a passivation agent in such a quantity that its equivalent oxygen content is at least 0.1%, preferably from 0.15 to 0.30% in moles with respect to the moles of carbon dioxide, with the formation of a first liquid mixture containing urea, ammonium carbamate, water and ammonia, in liquid/vapour equilibrium with a gaseous phase containing at least a part of the passivation agent; (ii) transferring said first liquid mixture to at least one decomposition-stripping step carried out in a suitable vertical apparatus, said stripper, operating at a pressure of at least 0.1 MPa, preferably from 0.2 to 2.0 MPa, lower than that of said reaction step (i); (iii) heating said first liquid mixture in said decomposition-stripping step, to effect the decomposition of part of the ammonium carbamate into ammonia and carbon dioxide, and simultaneously subjecting said liquid mixture to stripping with the formation of a first gaseous mixture containing ammonia and carbon dioxide, and a second liquid mixture containing urea, water, ammonia and the non-decomposed part of the ammonium carbamate; (iv) transferring at least a part, preferably all, of said first gaseous mixture to at least one condensation step, operating at the same pressure as the decomposition-stripping step (iii) and condensing the gaseous mixture transferred with the formation of a third liquid mixture containing ammonium carbamate, water and ammonia; (v) transferring said third liquid mixture and the possible remaining part of said first gaseous mixture to the reaction step (i); characterized in that at least a part of said gaseous phase in equilibrium with the first liquid mixture of step (i), is separated in a gas-liquid separator situated downstream of the reactor and operating at the same pressure, to form a second gaseous mixture containing a passivation agent, which is fed to the lower section of said stripper. (i) reacting ammonia and carbon dioxide at an overall pressure ranging from 12 to 20 MPa, with a molar ratio NH 3 /CO 2, as such or in the form of ammonium carbamate, ranging from 2.1 to 6, preferably from 2.8 to 4.5, in a reaction step carried out in a suitable vertical reactor R fed with at least one ammonia stream and at least one stream of fresh carbon dioxide containing a passivation agent in such a quantity that its equivalent oxygen content is at least 0.1%, preferably from 0.15 to 0.30% in moles with respect to the moles of carbon dioxide, with the formation of a first liquid mixture containing urea, ammonium carbamate, water and ammonia, in liquid/vapour equilibrium with a gaseous phase containing at least a part of the passivation agent; (ii) transferring said first liquid mixture to at least one decomposition-stripping step carried out in a suitable vertical apparatus, said stripper, operating at a pressure of at least 0.1 MPa, preferably from 0.2 to 2.0 MPa, lower than that of said reaction step (i); (iii) heating said first liquid mixture in said decomposition-stripping step, to effect the decomposition of part of the ammonium carbamate into ammonia and carbon dioxide, and simultaneously subjecting said liquid mixture to stripping with the formation of a first gaseous mixture containing ammonia and carbon dioxide, and a second liquid mixture containing urea, water, ammonia and the non-decomposed part of the ammonium carbamate; (iv) transferring at least a part, preferably all, of said first gaseous mixture to at least one condensation step, operating at the same pressure as the decomposition-stripping step (iii) and condensing the gaseous mixture transferred with the formation of a third liquid mixture containing ammonium carbamate, water and ammonia; (v) transferring said third liquid mixture and the possible remaining part of said first gaseous mixture to the reaction step (i);