Cabinet minutes detailing a plan to shelve the introduction of child benefit by James Callaghan's government were leaked by former minister Malcolm Wicks, his posthumous memoirs revealed.
The disclosure in 1976 shocked Westminster and led to an extensive leak inquiry but Mr Wicks, who was then a junior civil servant, escaped detection and went on to become a Labour MP and government minister.
The documents revealed that then prime minister Mr Callaghan had manipulated the Cabinet in an attempt to force the abandonment of the pledge to introduce child benefit.
Mr Wicks, who died aged 65 in 2012, said he felt it was a "moral issue" and his actions have been credited with saving child benefit as the Callaghan government was forced to perform a U-turn and reinstate the policy.
The papers were passed to Frank Field, who at the time was head of the Child Poverty Action Group (CPAG) but later also went on to be a Labour MP and minister.
Writing in his book, My Life, Mr Wicks defended his actions: "Was I right to leak the Cabinet papers? I still think I was," he writes.
"In the normal course of events civil servants, ministers and special advisers should not leak confidential material. It goes without saying that matters relating to national security have to be heavily safeguarded.
"But regarding the introduction of child benefit there was, I felt, a moral issue. It simply could not be right that ministers, at the most senior level, should manipulate internal discussions in such a way that the Cabinet itself was misled. I thought - and still think - that in those circumstances it was justifiable to leak or, putting it more positively, to let the wider public know what was going on."
In extracts of the book published in The Guardian Mr Wicks, who served as MP for Croydon North, explained why he was so outraged.
"As days passed and I saw more documentation, including Cabinet papers, it was not so much the attempt to abandon child benefit that incensed me, but more the way it was being done: the manoeuvring, the downright lies and the attempt to play off Labour MPs against trade union bigwigs," he wrote.
"My view was that if a Labour government was to abandon its policy, having connived and misled, then I had a duty to leak what happened to the papers - knowing full well that this would have repercussions - so that people would see the truth."
The papers were passed to Mr Field, who broke the story in an article in the magazine New Society and dubbed his source "Deep Throat" in a reference to the Watergate scandal.
In the foreword to the memoirs Mr Field said only a "tiny handful" of people had known the secret.
"Without the leak of Cabinet papers, it is entirely possible that child benefit would never have been introduced," he said.
"Malcolm ensured that some truly shameful manoeuvring within the Callaghan Cabinet to ditch one of Labour's central manifesto pledges was exposed.
"Through what the Observer newspaper went on to describe as 'the most extensive leak of cabinet papers [in a] century', he also made sure the CPAG was in poll position to pressure the Government into a further U-turn to get itself back on track.
"The biggest-ever redistribution of income to families, and particularly poor families was thereby secured - and without Malcolm's leak none of this redistribution would have taken place".