A catalytic system based at least on a preformation conjugated diene monomer, on an organometallic compound as cocatalyst and on a metallocene of formula {P(Cp1)(Cp2)Y} is provided. In the formula, Y denotes a group comprising a metal atom which is a rare earth metal, Cp1 and Cp2, which are identical or different, are selected from the group consisting of fluorenyl groups, cyclopentadienyl groups and indenyl groups, the groups being substituted or unsubstituted, and P is a group bridging the two Cp1 and Cp2 groups and comprising a silicon or carbon atom. Such a catalytic system exhibits an improved stability of the catalytic activity over time, in particular on storage.