A method for preparing carbon nanotubes, nanofibres or nanofilaments by decomposition of at least one carbon precursor in the presence of a catalyst, in which method continuously:—a first gas stream comprising a precursor of a catalyst is brought into contact with a porous substrate (43);—a second gas stream comprising at least one carbon precursor is brought into contact with said porous substrate (43);—said porous substrate (43) is heated to a temperature leading to the deposition of catalyst particles and to the catalytic growth of carbon nanotubes.