Catalytic solid for the (co)polymerisation of at least one olefin, comprising a coprecipitate of magnesium and of at least one transition metal, obtained by means of a process comprising the preparation of a mixture of a magnesium compound, such as the chloride or a magnesium alcoholate, and of a compound denoted by one of the formulae MY.sub.x (O--R').sub.t-x and M'O.sub.y (O--R").sub.s-2y and the treatment of the resulting mixture with a complex of formula M"(A)Al.sub.2 (X',X").sub.8 where X' and X" denote halogens, M and M' transition metals of groups IVB and VB, M" a transition metal of group IVB, A an aromatic hydrocarbon, Y a halogen or a group (O--R'"), and R', R" and R'" an alkyl, aryl or cycloalkyl group.