Kwasi Kwarteng claims he told Liz Truss to "slow down" after the market crashing mini-budget was announced.
The ex-chancellor, who was sacked by the then-prime minister, told TalkTV's First Edition programme: "She said, 'Well, I've only got two years' and I said, 'You will have two months if you carry on like this'."
Mr Kwarteng accepted he was "responsible" for the fiscal plan and that he was "not gonna wash my hands [of] it".
However, he refused to apologise for what happened, saying instead: "Looking back, I think we could have had a much more measured approach."
The Tory MP also confirmed rumours that he found out about his sacking on Twitter, and said that when he arrived at Number 10, Ms Truss was "distressed and emotional".
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'She was very forceful'
Asked by presenter Tom Newton Dunn what went wrong, the MP said: "So I think we tried to do too much too quickly. Too much, too fast. I think the strategic goal was right.
"But I think we should have had a much more measured approach, because we could have had, as we're having an autumn statement. And of course, it'll be in the budget in April.
"So I think that was, you know, her vision. Her drive was 100% the right thing, but I think we needed a better tactical plan to deliver what she wanted."
Mr Kwarteng continued to point the finger squarely at Ms Truss, saying she was "very much of the view that we needed to seize the opportunity, and we hit the ground running", adding: "She's very dynamic, very forceful. That's a great strength."
But he said "the lesson that we've learned" is to have "a tactical and measured approach".
Asked who was responsible for the pace, he again said the former PM, who was "very much of the view that we needed to move things fast", adding: "But I think it was too quick."
However, he admitted he did not tell Ms Truss that at the time, telling the programme: "Well, I said actually after the budget that because we were going very fast even after the mini-budget we were going at breakneck speed. And I said, 'You know, we should slow down, slow down'.
"She said, 'Well, I've only got two years' and I said, 'You will have two months if you carry on like this'."
Asked several times to apologise, Mr Kwarteng instead said: "I do feel sorry actually for the people who are going through this difficult time in terms of re-mortgaging. I think that it is a really stressful thing to do. And I really feel sympathy for that."
The long-term political ally was brought into the Treasury by Ms Truss as her first cabinet appointment after winning the Conservative leadership contest over the summer.
In September, he announced a raft of unfunded tax cutting policies in the Commons that sent the markets into a tailspin.
Initially the pair stuck to their plans, but after a number of U-Turns, Mr Kwarteng was ousted by his boss and replaced by the current Chancellor Jeremy Hunt, who within days reversed all the plans he had set out.
Days later, Ms Truss resigned from office.
'Buy her more time'
Talking about his sacking, Mr Kwarteng said Ms Truss was "very emotional", adding: "I can't remember whether she was actually shedding tears, but she was very emotional, and it was a difficult thing to do.
"And I think she genuinely thought that that was the right thing to buy her more time to set her premiership on the right path.
"I disagreed obviously, because I thought that if chancellors are sacked by the prime minister for doing what the prime minister campaigned on, that leaves the prime minister in a very weak position. And a number of people have said to me, 'I knew when she'd sack you that it was over'."
But he insisted the pair were still friends, despite having missed a call from her "a couple of days ago" and not yet returning it.
Ending the interview, he praised the new PM Rishi Sunak, saying he was "actually very credible prime minister" and was "very cool and very considered".
But ahead of the autumn statement next week, the ex-chancellor issued a warning that he still maintains the belief "you're not going to grow an economy, or incentivise economic growth, by putting up our taxes".