This invention relates to a polyester urethane hybrid resin casting resin composition which comprises a mixture ABCD of an unsaturated polyester ABC which has moieties derived from at least one alcohol A, moieties derived from a mixture B of carboxylic acids, wherein at least 10 % of the mass of at least one of the said carboxylic acids B or of the said alcohols A have at least one olefinic unsaturation in their molecules, and moieties derived from a monoepoxide C having at least four carbon atoms, which polyester ABC is dissolved in a compound D, and which has at least one olefinic unsaturation and which is radically copolymerisable with the unsaturated polyester oligomer ABC and a second liquid component EF comprising a polyfunctional isocyanate E and a radical initiator F, a process for its production, and a method of use thereof to form large moulded parts.