Lord Mandelson has warned he will "ruffle some feathers" by publishing the inside story of the New Labour "soap opera" this summer.
The former Deputy Prime Minister has promised to lift the lid on what really went on between Tony Blair and Gordon Brown and said the book will offer a "mixture of history, autobiography and emotion".
He also revealed he has regrets over helping create the pair's long-running feud by persuading Mr Brown not to seek the leadership in 1994.
"After (former Labour leader) John Smith died I was mistaken in arguing so hard that the two modernising candidates should not oppose each other," Lord Mandelson said. "I did so from the best motives. I did not want two friends to hurt each other...I did not want the modernisers' cause damaged with the risk that a split vote might let in someone else...But if we had resolved the matter there and then, we would have avoided so much of the soap opera that followed."
Lord Mandelson said electoral defeat killed off the era of politics he helped create.
"This phase of New Labour is now over and died on May 6, 2010," he said.