The present invention relates to a catalytic system usable for the preparation of polybutadienes by polymerisation, to a process for the preparation of said catalytic system and to a process for the preparation of polybutadienes by means of this catalytic system.A catalytic system according to the invention is based on at least: a conjugated diene monomer, an organic phosphoric acid salt of one or more rare earth metals, said salt being in suspension in at least one inert, saturated and aliphatic or alicyclic hydrocarbon solvent, an alkylating agent consisting of an alkylaluminium of formula AlR.sub.3 or HAlR.sub.2, the "alkylating agent:rare earth salt" molar ratio being greater than 5, and a halogen donor which belongs to the family of alkylaluminium halides with the exception of alkylaluminium sesquihalides, and, according to the invention, said catalytic system comprises said rare earth metal(s) in a concentration equal to or greater than 0.005 mol/l.