Former Labour cabinet minister Patricia Hewitt defended a proposal to lower the age of consent in the face of a school teacher's accusation that she was seeking to "shatter prospective individual happiness at an early age".

The then general secretary of the National Council for Civil Liberties was writing in April 1976 in response to a letter from a teacher at St Paul's boys' school in London. He had accused the organisation of having "some very twisted minds" behind it.

Hewitt wrote in her letter: "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."

She was responding to Philip McGuinness, a house master at St Paul's, a leading public school, who wrote to the NCCL on 14 March that year expressing his disgust.

A month earlier Hewitt's name had appeared on an NCCL press release that proposed cutting the age of consent to 14 and in some circumstances 10.

In the correspondence discovered by the Guardian in the NCCL's archives in Hull, McGuinness wrote: "I cannot help but think that you do not support civil liberties at all. Your aim is questionable in the extreme. 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?"

He signed off by saying: "Your title is a shame and a masquerade. There must be some very twisted minds and pernicious malcontents behind your organisation if this is the sort of thing you advocate."

The March 1976 NCCL press release said: "NCCL proposes that the age of consent should be lowered to 14, with special provision for situations where the partners are close in age, or where consent of a child over ten can be proved."

The release relates to an NCCL report on sexual law reforms. In it Hewitt also said: "The report argues that the crime of incest should be abolished. It says, 'In our view, no benefit accrues to anyone by making incest a crime when committed between mutually consenting persons over the age of consent'."

In a statement released to media on Thursday night, Hewitt insisted: "The proposal to reduce the age of consent was not mine – it was the policy of the organisation and its executive committee at that time. I do not support reducing the age of consent or legalising incest." She did not respond to requests for comment on Friday.

Hewitt, who now sits on boards at BT and Bupa, is one of three Labour figures who were leading lights in the NCCL in the 1970s at a time when the Paedophile Information Exchange was affiliated to the organisation and some of its members attempted to influence NCCL policy. Labour deputy leader Harriet Harman was the organisation's legal officer during a later period between 1978 and 1982, while her husband Jack Dromey was on the executive from 1970 to 1979.According to archives held at Hull University, in December 1975 Keith Hose, chairman of PIE, wrote to Patricia Hewitt, then general secretary of NCCL and later a Labour health secretary, asking her to consider PIE's views in its policy on ages of consent. The letter was on PIE notepaper which features a logo of two bare-legged children sitting on a rock. Hewitt wrote back saying: "We have found your evidence ... most helpful".

Earlier on Friday, Dromey, insisted he did not give his approval to the NCCL's call for the age of consent to be reduced to as low as 10, after the Sun reported that he had attended an executive committee in January 1976 where the change was discussed.

Dromey said he did not give his agreement to the proposal at the committee meeting a month earlier, and was "a resolute opponent" of the PIE when he became chairman a few weeks later.

"I did not agree with the proposal in February 1976 to lower the age of consent," he said. "When elected chairman of NCCL weeks later, I made it clear that my first priority would be to take on the child sex abusers of PIE. I then defeated them by a massive majority at the annual conference in April."

Aides to Harman meanwhile said that she stood behind her decision not to apologise for the NCCL's relationship to PIE after Patricia Hewitt's apology. The aide said Harman fully backs Hewitt's apology for being "naive and wrong" about PIE and was in touch with her before the statement was made. Harman is also still furious with the Daily Mail for labelling her an apologist for paedophiles and continues to believe herself to be the subject of a political smear. A Labour source said they believed Hewitt was right to apologise but Harman has no reason to do so.