Process for the preparation of a solid catalyst for the (co)polymerisation of α-olefins, comprising titanium, magnesium, aluminium, chlorine and, optionally, at least one metal M selected from hafnium and zirconium, preferably zirconium, comprising the following steps in succession: (i) preparing a first mixture by heating in a hydrocarbon liquid medium to a temperature from 40 to 200°C, preferably from 60 to 130°C, for at least one minute, preferably for a time between 0.5 and 20 hours, at least one magnesium chloride, at least one titanium compound, at least one carboxylic acid R-COOH, wherein R is an organic group having from 2 to 30 carbon atoms, preferably from 5 to 15 carbon atoms, and, optionally, at least one compound of said metal M, in an amount such as to comply with the following ranges of atomic or molar ratios: M/Ti = 0.0-5.0; Mg/Ti = 3.0-15.0; R-COOH/(Mg + M) = 1.5-8 (ii) adding to the first mixture obtained in step (i), preferably at a temperature between 10 and 70°C, a chlorinating agent selected from a silicon chloride and an aluminium chloride which are soluble in said hydrocarbon liquid medium, in a sufficient quantity to cause at least 70%, preferably at least 80%, of the magnesium and any metal M present in the solution to precipitate in the form of solid chlorides, so as to obtain a second mixture comprising a liquid phase and a solid phase; (iii) adding to the second mixture obtained in the previous step (ii) an organometallic compound of a metal M' having the following formula: M'R"" n Cl (p-n) wherein: M' is a metal selected from trivalent aluminium, trivalent gallium, tetravalent tin or divalent zinc; R"" is a linear or branched alkyl radical containing from 1 to 20 carbon atoms, "n" is a decimal number having a value between 0.5 and p, preferably between 1 and p, where p is the valence of M'; and reacting until at least 70%, preferably at least 80%, of the titanium present has precipitated in the form of a solid compound to obtain said solid catalyst. (i) preparing a first mixture by heating in a hydrocarbon liquid medium to a temperature from 40 to 200°C, preferably from 60 to 130°C, for at least one minute, preferably for a time between 0.5 and 20 hours, at least one magnesium chloride, at least one titanium compound, at least one carboxylic acid R-COOH, wherein R is an organic group having from 2 to 30 carbon atoms, preferably from 5 to 15 carbon atoms, and, optionally, at least one compound of said metal M, in an amount such as to comply with the following ranges of atomic or molar ratios: M/Ti = 0.0-5.0; Mg/Ti = 3.0-15.0; R-COOH/(Mg + M) = 1.5-8 (ii) adding to the first mixture obtained in step (i), preferably at a temperature between 10 and 70°C, a chlorinating agent selected from a silicon chloride and an aluminium chloride which are soluble in said hydrocarbon liquid medium, in a sufficient quantity to cause at least 70%, preferably at least 80%, of the magnesium and any metal M present in the solution to precipitate in the form of solid chlorides, so as to obtain a second mixture comprising a liquid phase and a solid phase; (iii) adding to the second mixture obtained in the previous step (ii) an organometallic compound of a metal M' having the following formula: M'R"" n Cl (p-n) wherein: M' is a metal selected from trivalent aluminium, trivalent gallium, tetravalent tin or divalent zinc; R"" is a linear or branched alkyl radical containing from 1 to 20 carbon atoms, "n" is a decimal number having a value between 0.5 and p, preferably between 1 and p, where p is the valence of M'; M' is a metal selected from trivalent aluminium, trivalent gallium, tetravalent tin or divalent zinc; R"" is a linear or branched alkyl radical containing from 1 to 20 carbon atoms, "n" is a decimal number having a value between 0.5 and p, preferably between 1 and p, where p is the valence of M';