A process for preparation of an extrusion-blow-moldable co-polyester resin composition; said process comprising the following steps: a. charging in a reactor at least one pair of polyester-forming materials selected from the group of pairs consisting of a 'diol-dicarboxylic acid' pair, a 'diol- dicarboxylic ester' pair and diol-recycled PET pair, along with a co-monomer and at least one additive selected from the group consisting of an impact modifier, an antioxidant, a catalyst, an acetaldehyde inhibitor and a color toner, to obtain a reaction mixture; b. subjecting the reaction mixture to a method step selected from the group consisting of esterification, ester-interchange reaction and glycolysation yielding a pre-polymer; c. charging a chain extender, in a proportion of 0.05 wt. % to 2.0 wt. %, preferably 0.05 wt. % to 0.8%, more preferably 0.05 wt. % to 0.4 wt. % in the reaction mixture during step (b) in at least one portion or in continuous manner at controlled dosing rate when the intrinsic viscosity (I.V) (as described in part "A] Determination of intrinsic viscosity" of the description) of the reaction mixture is <0.20, preferably <0.10; d. subjecting the pre-polymer to polycondensation at a temperature in the range of 270 °C to 305 °C under pressure of less than 1000 Pa (10 mb = 10 mbar) preferably below 200 Pa (2 mb = 2 mbar) more preferably below 100 Pa (1 mb = 1 mbar) to obtain amorphous chips of I.V in range 0.40 to 0.80; e. crystallizing the amorphous chips at a temperature in the range of 110°C - 170°C to obtain chips with a crystallinity (as described in part "G] Determination of crystallinity" of the description) of more than 30%; and f. processing the crystallized chips in a Solid State Polymerizer at a temperature in the range of 190 °C to 225°C till the required I.V of 0.70 to 2.0, preferably 0.90 to 1.50, more preferably 0.90 to 1.40 is achieved to obtain an extrusion blow molding grade co-polyester resin composition. a. charging in a reactor at least one pair of polyester-forming materials selected from the group of pairs consisting of a 'diol-dicarboxylic acid' pair, a 'diol- dicarboxylic ester' pair and diol-recycled PET pair, along with a co-monomer and at least one additive selected from the group consisting of an impact modifier, an antioxidant, a catalyst, an acetaldehyde inhibitor and a color toner, to obtain a reaction mixture; b. subjecting the reaction mixture to a method step selected from the group consisting of esterification, ester-interchange reaction and glycolysation yielding a pre-polymer; c. charging a chain extender, in a proportion of 0.05 wt. % to 2.0 wt. %, preferably 0.05 wt. % to 0.8%, more preferably 0.05 wt. % to 0.4 wt. % in the reaction mixture during step (b) in at least one portion or in continuous manner at controlled dosing rate when the intrinsic viscosity (I.V) (as described in part "A] Determination of intrinsic viscosity" of the description) of the reaction mixture is <0.20, preferably <0.10; d. subjecting the pre-polymer to polycondensation at a temperature in the range of 270 °C to 305 °C under pressure of less than 1000 Pa (10 mb = 10 mbar) preferably below 200 Pa (2 mb = 2 mbar) more preferably below 100 Pa (1 mb = 1 mbar) to obtain amorphous chips of I.V in range 0.40 to 0.80; e. crystallizing the amorphous chips at a temperature in the range of 110°C - 170°C to obtain chips with a crystallinity (as described in part "G] Determination of crystallinity" of the description) of more than 30%; and f. processing the crystallized chips in a Solid State Polymerizer at a temperature in the range of 190 °C to 225°C till the required I.V of 0.70 to 2.0, preferably 0.90 to 1.50, more preferably 0.90 to 1.40 is achieved to obtain an extrusion blow molding grade co-polyester resin composition.