An intermediate polymer preform of elongated shape for manufacturing a microstructured optical fibre is made from a mould with a tubular par containing a pattern of hole generating elements (wires, rods or tubes) extending along it. The hole generating elements are releasably attached to an upper and to a lower base of the mould. The mould is cleaned by circulating a filtered solvent in it. The mould is filled with a fluid polymer optical material or polymer precursor. The polymer material or precursor is consolidated so that it cannot change its shape under operating stress conditions at ambient temperature. The hole generating elements are released from the upper and lower base. The consolidated core preform is extracted from the mould. The process can further include calibrating the intermediate preform.