Tony Blair's youthful enthusiasm for radical socialism and his admiration for socialist theorist Karl Marx are revealed in letter written in 1982.
Mr Foot took over the leadership of Labour from Jim Callaghan
In the 22-page letter, the 29-year-old Mr Blair tells then Labour leader Michael Foot how reading Marx had "irreversibly altered" his outlook.
He also praises Tony Benn, agreeing with the left-winger's analysis that Labour's right-wing was bankrupt.
Details of the future prime minister's letter are in the New Statesman.
Writing just after he had lost Labour's deposit as the party's candidate in the Beaconsfield by-election, Mr Blair describes how he was initially inspired by Marx but later rejected his teachings.
"I actually did trouble to read Marx first hand. I found it illuminating in so many ways; in particular, my perception of the relationship between people and the society in which they live was irreversibly altered," he wrote.
"But ultimately it was stifling because it sought to embrace in its philosophy every facet of existence. That, of course, is its attraction to many."
Mr Blair also attacked the "arrogance and self-righteousness" of many groups on the far left which he argued were "deeply unattractive" to ordinary Labour members.
The future prime minister believed Mr Foot should expel Militant Tendency - something that Neil Kinnock would ultimately achieve.
"There should not be a party within a party," he said.
'Different approach needed'
Mr Blair went on to urge Mr Foot to adopt bold policies to convince the "legitimate left" that he had not become a "prisoner of the right".
"I would indicate firmly that you believe the party needs radical, socialist policies; that the scale of the problems we face as a nation in 1982 means a different approach to previous years," he wrote.
"The job of reconstruction, particularly against a background that includes new technology and a USA in the grip of the same economic madness Mrs Thatcher visits upon us, is mammoth.
"Profound problems require profound remedies."
Mr Blair signs off advising Mr Foot to make clear he would be leading Labour into the next election and that he would win it.
In fact Mr Foot suffered a resounding defeat at the hands of Margaret Thatcher in the 1983 poll.
The letter was discovered by historian Robert Taylor while he was researching a history of the Parliamentary Labour Party.