The best-selling author, a once close confidant of the former prime minister who has since become a fierce critic, lashed out at Blair for turning his back on parliament once he quit as PM to go and "hang out with a lot of rich people in America".
In his latest attack on the former Labour leader, who has never escaped the legacy of Iraq, Harris also holds up Lord Mandelson, who has been known to associate with a super-rich crowd, as a paragon of plain living and frugality compared to ex-premier.
Harris, a former political journalist, is the author of The Ghost, a book turned into a film about a former British prime minister who faces being hauled in front of the International Criminal Court for alleged war crimes, which he previously described as being somewhere "between reality and fiction".
Asked about Blair's life today, he told Total Politics: "I find it tragic. I think it's presumptuous of me to say so but I can't believe there isn't an element of tragedy that he himself feels, that a relatively young man in political terms should cut himself off from British democracy in the way that he has, because he could have had one of those 19th century careers and come back, as foreign secretary or maybe even as party leader, but he turned his back on it and walked out of the place.
British journalist and author Robert Harris
"I cannot understand someone who tasted that kind of role and fought for it, turning their back on it."
He added: "Who knew that he would become a great friend of George Bush and would want to keep bombing people and would be so passionately interested in making money and live this strange life with the billionaire super-rich on yachts and private jets?
"I mean maybe someone more perceptive than I would have seen it, but I never saw that at the time, nor - knowing a lot of the people who know him very well - did they.
"It's a cliche to say that most politicians go mad if they are in office for more than about six or seven years and they become a member of a club and you become quite disconnected from reality, and I think there were in Tony things we perhaps didn't realise at the time - of narcissism, a messiah complex, that had merely accelerated this impulse in him."
Harris said Lord Mandelson, godfather to one of his children, still mixed with the "paupers".
"Peter is the soul really of plain living - frugality - compared to Tony Blair as far as I can see. He associates with the paupers of the earth compared to the people that Blair associates with," he told the magazine.
Blair's recent pointed interventions into European politics have triggered speculation that he is angling for a top job in Brussels, perhaps even president of the European Commission.
However the former prime minister insisted on Monday morning that he was not a candidate. "Don't get my position mixed up with this, that's not going to happen," he said.
The clarification was a relief to Ukip MEP Jane Collins, who told BBC Two's Daily Politics programme: "I'm so relieved to hear Tony Blair say he is not going to be a candidate, because quite frankly he is a megalomanic and a warmonger."
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