A catalytic system usable for obtaining a copolymer of a conjugated diene and at least one monoolefin such as an alpha-olefin and/or ethylene, characterised in that it comprises: (i) an organometallic complex represented by the following generic formula: {[P(Cp)(Fl)Ln (L x)} p where Ln represents a lanthanide atom of an atomic number from 57 to 71, to which is attached a ligand molecule comprising cyclopentadienyl Cp and fluorenyl Fl groups which are each independently substituted or unsubstituted and linked to one another by a bridge P of the formula MR 1 R 2, where M is an element from column IVa of Mendeleev's periodic table and where R 1 and R 2, which may be identical or different, each represent alkyl groups having from 1 to 20 carbon atoms or alternatively cycloalkyl or phenyl groups having from 6 to 20 carbon atoms, where X represents a halogen atom which may be chlorine, fluorine, bromine or iodine, where L comprises an optional complexing molecule, such as an ether, and optionally a substantially less complexing molecule, such as toluene, where p is a natural integer greater than or equal to 1 and x is greater than or equal to 0, and (ii) a co-catalyst belonging to the group consisting of alkylmagnesiums, alkyllithiums, alkylaluminiums, Grignard reagents and mixtures of these constituents. (i) an organometallic complex represented by the following generic formula: {[P(Cp)(Fl)Ln (L x)} p where Ln represents a lanthanide atom of an atomic number from 57 to 71, to which is attached a ligand molecule comprising cyclopentadienyl Cp and fluorenyl Fl groups which are each independently substituted or unsubstituted and linked to one another by a bridge P of the formula MR 1 R 2, where M is an element from column IVa of Mendeleev's periodic table and where R 1 and R 2, which may be identical or different, each represent alkyl groups having from 1 to 20 carbon atoms or alternatively cycloalkyl or phenyl groups having from 6 to 20 carbon atoms, where X represents a halogen atom which may be chlorine, fluorine, bromine or iodine, where L comprises an optional complexing molecule, such as an ether, and optionally a substantially less complexing molecule, such as toluene, where p is a natural integer greater than or equal to 1 and x is greater than or equal to 0, and where Ln represents a lanthanide atom of an atomic number from 57 to 71, to which is attached a ligand molecule comprising cyclopentadienyl Cp and fluorenyl Fl groups which are each independently substituted or unsubstituted and linked to one another by a bridge P of the formula MR 1 R 2, where M is an element from column IVa of Mendeleev's periodic table and where R 1 and R 2, which may be identical or different, each represent alkyl groups having from 1 to 20 carbon atoms or alternatively cycloalkyl or phenyl groups having from 6 to 20 carbon atoms, where X represents a halogen atom which may be chlorine, fluorine, bromine or iodine, where L comprises an optional complexing molecule, such as an ether, and optionally a substantially less complexing molecule, such as toluene, where p is a natural integer greater than or equal to 1 and x is greater than or equal to 0, and (ii) a co-catalyst belonging to the group consisting of alkylmagnesiums, alkyllithiums, alkylaluminiums, Grignard reagents and mixtures of these constituents.