The present invention concerns the field of fluid catalytic cracking (FCC) processes. The invention provides a process increasing production of LPG and propene in FCC units operating under conditions of maximisation of middle distillates of low aromaticity, such that they may be incorporated into the diesel oil pool. The invention also relates to the preparation and employment of additives based on zeolites having small pores, such as ferrierite (FER), in catalytic systems for FCC units, wherein conditions of low severity are adopted with a view to increasing yields of LPG and light olefins whilst improving stability of petrol. The invention also provides an original catalytic system, being more efficient than catalytic systems known in the state of the art, to increase the yield of LPG and propene without prejudicing the yield and quality of LCO. Furthermore it provides a method of preparation of an additive for said process employing the zeolite ferrierite.