The prime minister provoked outrage with the description of militant miners who resisted pit closures delivered in a speech for the 1922 Committee in July 1984.
Her personal papers from that year - which contain the first handwritten draft of The Enemy Within speech - reveal that she had no plans to back down.
In fact a scribbled speech, released for the first time by the Margaret Thatcher Archive Trust, shows she planned to go further by launching a rare full frontal attack on the Labour Party.
But this speech was torn up, only to be taped back together by her own hand for posterity, after the terrorist attack on the Grand Hotel, Brighton, which killed five people at the start of the Conservative Party conference on October 12 that year.
Revealed for the first time, the speech begins: “From the dark cloud falls an acid rain that eats into liberty.”
Mrs Thatcher goes on to attack Labour’s support of the National Union of Mineworkers and unwillingness to condemn picket violence.
The speech says: “The Labour Party in its present form, infilitrated by extremists, riven by factions, still stands upon the stage as the (principal) alternative to the Conservative Party in governing Britain.
“That, Mr Chairman, is the measure of the shadow which has fallen across freedom since last we met.”
Chris Collins, from the Margaret Thatcher Foundation, is the only historian to have studied the papers to date and which have been published today for the first time.
He said Mrs Thatcher had been forced to soften her stance as a result of the national mood in the wake of the bombing - stepping back from a moment which would otherwise have helped define her career.
Mr Collins said: “It was a speech which would have been remembered as controversial and would have eclipsed The Enemy Within speech - indeed it was intended to do that.
“She would have taken some convincing to deliver such an attack on Labour and only done so within extreme provocation as such partisan politics were against her nature.
“But tragic events overtook her and, instead of a moment which would have been remembered for decades, the speech ended up torn up and later taped back together - probably by her own hand as she was a dab hand with Sellotape.”
The speech was eventually delivered and did contain a few carefully chosen fragments of its more controversial predecessor but with virtually no references to the Labour Party.
Mr Collins said: “This was softening to fit the national mood but also a genuine softening of her own stance.
“In the days that followed the bombings she received hundreds of letters of support - including from opposition politicians - which genuinely moved her.”
He added: “It is ironic that the speech is softened by an act of great violence.”