A method for manufacturing an optical fibre preform comprising: - providing a furnace comprising a furnace tube having an elongated heating chamber configured to house a preform, the heating chamber having an upper entry opening for the insertion of the preform; - providing an extension tube comprising an extension chamber having a lower opening and configured to house at least a length portion of a preform; - inserting a lower length portion of a porous optical fibre preform in the heating chamber in such a way that an upper length portion of the porous preform lies outside of the furnace tube; - enclosing the upper length portion of the optical fibre preform in the extension tube; - joining the furnace tube with the extension tube so as the lower opening of the extension chamber is in communication with the upper entry opening of the heating chamber to form a single chamber; - dehydrating the lower length portion of the preform by exposing said lower length portion to a first heating zone of the furnace tube set at a dehydration temperature, characterised in that the furnace tube has a second heating zone positioned below the first heating zone and set at a consolidation temperature, the method comprising - gradually lowering the preform through the first heating zone so as to dehydrate the upper length portion of the optical fibre preform, wherein gradually lowering the optical fibre preform comprises exposing successive length portions of the preform to the first heating zone and subsequently to the second heating zone until the whole preform has passed through the first heating zone, and after the optical fibre preform has passed through the first heating zone, continuing to gradually lower the optical fibre preform through the second heating zone until the whole preform has passed through the second heating zone so as to consolidate the whole optical fibre preform. - providing a furnace comprising a furnace tube having an elongated heating chamber configured to house a preform, the heating chamber having an upper entry opening for the insertion of the preform; - providing an extension tube comprising an extension chamber having a lower opening and configured to house at least a length portion of a preform; - inserting a lower length portion of a porous optical fibre preform in the heating chamber in such a way that an upper length portion of the porous preform lies outside of the furnace tube; - enclosing the upper length portion of the optical fibre preform in the extension tube; - joining the furnace tube with the extension tube so as the lower opening of the extension chamber is in communication with the upper entry opening of the heating chamber to form a single chamber; - dehydrating the lower length portion of the preform by exposing said lower length portion to a first heating zone of the furnace tube set at a dehydration temperature, characterised in that the furnace tube has a second heating zone positioned below the first heating zone and set at a consolidation temperature, the method comprising - gradually lowering the preform through the first heating zone so as to dehydrate the upper length portion of the optical fibre preform, wherein gradually lowering the optical fibre preform comprises exposing successive length portions of the preform to the first heating zone and subsequently to the second heating zone until the whole preform has passed through the first heating zone, and after the optical fibre preform has passed through the first heating zone, continuing to gradually lower the optical fibre preform through the second heating zone until the whole preform has passed through the second heating zone so as to consolidate the whole optical fibre preform.