A catalyst precursor for the (co)polymerization of alpha-olefins, comprising titanium, magnesium, aluminium, chlorine and at least one metal M selected from hafnium and zirconium, preferably hafnium, characterized in that it is obtained by means of a process comprising the following steps: (i) heating a mixture in a liquid hydrocarbon medium comprising a magnesium chloride, a compound of titanium, a compound of said metal M, a carboxylic acid R-COOH, wherein R is an organic group having from 2 to 30 carbon atoms, in such quantities as to respect the following atomic or molar ratio ranges: M/Ti = 0.2-5.0; Mg/Ti = 3.0-20.0; R - COOH/(Mg +M) = 1-8 at a temperature ranging from 50 to 200°C, preferably from 60 to 130°C, for at least one minute, preferably for a time ranging from 0.5 to 4 hours, separating the possible solid residue remaining undissolved, to obtain a solution; (ii) adding to the solution obtained in step (i), an aluminium alkyl chloride having the following general formula: AlR' n Cl (3-n) wherein: R' is a linear or branched alkyl radical, containing from 1 to 20 carbon atoms, and "n" is a decimal number having values ranging from 0.5 to 2.5, preferably from 0.9 to 2.1, in an amount sufficient for precipitating in the form of a solid compound at least 70%, preferably at least 80%, of the metals Mg, M and Ti present in said solution, and heating the mixture thus obtained to a temperature ranging from 40 to 130°C, preferably from 60 to 110°C, for a time ranging from 5 to 240 minutes, preferably from 40 to 150 minutes, to obtain a solid precipitate comprising Mg, M, Al and Cl in atomic ratios with respect to the Ti within the following ranges: M/Ti = 0.2-5.0; Mg/Ti = 3.0-15.0; Al/Ti = 0.1-4.0; Cl/Ti = 15.0-60.0; (iii) separating the solid precipitate thus formed from the residual liquid solution; (iv) putting said solid precipitate obtained in step (iii) in contact with a siloxane compound A, having from 2 to 40 carbon atoms and from 1 to 15 silicon atoms, comprising at least one siloxane group selected from the following formulae: O-Si-C and Si-O-Si, in such an amount that the atomic ratio Si/Ti between the Si atoms in the siloxane compound A and the Ti atoms in the solid precipitate, is higher than or equal to 0.1, preferably ranging from 0.2 to 2.0, to obtain said catalyst precursor. (i) heating a mixture in a liquid hydrocarbon medium comprising a magnesium chloride, a compound of titanium, a compound of said metal M, a carboxylic acid R-COOH, wherein R is an organic group having from 2 to 30 carbon atoms, in such quantities as to respect the following atomic or molar ratio ranges: M/Ti = 0.2-5.0; Mg/Ti = 3.0-20.0; R - COOH/(Mg +M) = 1-8 at a temperature ranging from 50 to 200°C, preferably from 60 to 130°C, for at least one minute, preferably for a time ranging from 0.5 to 4 hours, separating the possible solid residue remaining undissolved, to obtain a solution; M/Ti = 0.2-5.0; Mg/Ti = 3.0-20.0; R - COOH/(Mg +M) = 1-8 at a temperature ranging from 50 to 200°C, preferably from 60 to 130°C, for at least one minute, preferably for a time ranging from 0.5 to 4 hours, separating the possible solid residue remaining undissolved, to obtain a solution; (ii) adding to the solution obtained in step (i), an aluminium alkyl chloride having the following general formula: AlR' n Cl (3-n) wherein: R' is a linear or branched alkyl radical, containing from 1 to 20 carbon atoms, and "n" is a decimal number having values ranging from 0.5 to 2.5, preferably from 0.9 to 2.1, in an amount sufficient for precipitating in the form of a solid compound at least 70%, preferably at least 80%, of the metals Mg, M and Ti present in said solution, and heating the mixture thus obtained to a temperature ranging from 40 to 130°C, preferably from 60 to 110°C, for a time ranging from 5 to 240 minutes, preferably from 40 to 150 minutes, to obtain a solid precipitate comprising Mg, M, Al and Cl in atomic ratios with respect to the Ti within the following ranges: M/Ti = 0.2-5.0; Mg/Ti = 3.0-15.0; Al/Ti = 0.1-4.0; Cl/Ti = 15.0-60.0; M/Ti = 0.2-5.0; Mg/Ti = 3.0-15.0; Al/Ti = 0.1-4.0; Cl/Ti = 15.0-60.0; (iii) separating the solid precipitate thus formed from the residual liquid solution; (iv) putting said solid precipitate obtained in step (iii) in contact with a siloxane compound A, having from 2 to 40 carbon atoms and from 1 to 15 silicon atoms, comprising at least one siloxane group selected from the following formulae: O-Si-C and Si-O-Si, in such an amount that the atomic ratio Si/Ti between the Si atoms in the siloxane compound A and the Ti atoms in the solid precipitate, is higher than or equal to 0.1, preferably ranging from 0.2 to 2.0, to obtain said catalyst precursor.