Notes from a 1983 meeting between then-home secretary Leon Brittan and a campaigning MP have revealed the peer objected to banning a paedophile rights group.
Documents obtained by the BBC, under Freedom of Information laws, also include a letter from Tory MP Geoffrey Dickens in which he said he "shall not rest" until the Paedophile Information Exchange (PIE) was outlawed.
Lord Brittan, who died in January, has been accused of being involved in child sex abuse after it emerged he had been questioned by police in 2014 in connection with an historical rape allegation.
And questions have been raised over his handling of the issue during his time in office as a 40-page dossier by the late Mr Dickens alleging high-level paedophile activity in Westminster, known as the "Dickens dossier", went missing.
In a letter, dated 17 August 1983, Mr Dickens, then MP for Littleborough and Saddleworth, asked for the government to take action against PIE.
Mr Dickens wrote: "I shall not rest until we outlaw such organisations and it will not embarrass me to put our front bench under tremendous pressure on this subject because this time I shall carry the House of Commons and the country with me - be in no doubt."
Minutes of a meeting on 23 November 1983 between Mr Dickens, Lord Brittan and several other officials reveal the peer told Mr Dickens he too sought "the protection of children" but was minded to see "how the present law stood".
But, according to the BBC, the minutes also record Lord Brittan's objection to a ban on the organisation, which eventually disbanded in 1984.
They read: "There was however the fundamental objection to banning an organisation that if members of the organisation were doing a mischief, it was the mischief which ought to be banned, not the organisation.
"It also has to be borne in mind that the number of people in PIE itself was small, but the number of people engaging in paedophilia was rather larger."
Mr Dickens is then said to have "indicated his agreement".
Having handed over two letters containing specific allegations against members of the Civil Service, he was told by Lord Brittan "any evidence of such activities which had substance to them would be investigated".
The Home Office admitted last July that more than 100 files relating to historic organised child abuse over a period of 20 years had gone missing.
The documents obtained by the BBC were among those examined in an independent review of Home Office files and the subsequent Wanless Inquiry, which failed to find the "Dickens dossier".
A wide-ranging independent inquiry, headed up by New Zealand Justice Lowell Goddard, has been established by the Home Secretary Theresa May to probe allegations of historical child sex abuse and a Westminster paedophile ring.