The allegation is the latest twist in an ongoing row over the activities of the National Council For Civil Liberties (NCCL) in the 1970s and the role played by Ms Hewitt and her former Cabinet colleague Harriet Harman.
Ms Hewitt, a former Health Secretary, had said she and Ms Harman were both "naive and wrong" to accept claims by the Paedophile Information Exchange that it was a campaigning and counselling organisation.
But last night a newspaper reported that Ms Hewitt, as the general secretary of the NCCL, responded in April 1976 to a letter from the house master of a London boys' school. Philip McGuinness had said the organisation had "some very twisted minds" in it after it released a statement - bearing Ms Hewitt's name - proposing the age of consent be cut to 14.
In her reply, from the NCCL's archives, Ms Hewitt reportedly wrote: "Our proposal that the age of consent be reduced is based on the belief that neither the police nor the criminal courts should have the power to intervene in a consenting sexual activity between two young people. It is clearly the case that a number of young people are capable of consenting to sexual activity and already do so."
Mr McGuinness reportedly responded: "Are you aiming for the destruction of society, for the enslavement of the individual, for the destruction of family life? Is your object to shatter prospective individual happiness at an early age?"
The release related to an NCCL report on sexual law reforms. Ms Hewitt has said she did not support its proposal to reduce consent.