A beyond the grave confession by former Croydon North MP Malcolm Wicks has solved one of the great political mysteries of the 20th century.

In 1976 he was the man who leaked Cabinet minutes detailing a plan to shelve the introduction of child benefit by James Callaghan's government, his posthumous memoirs have revealed.

The disclosure 38 years ago 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 of cancer 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."

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.