Mr Blair described Mr Brown as "flawed, lacking perspective and having a paranoia about him", during their fractious relationship at the head of British politics, he claimed.
According to the former business secretary Mr Blair added that Mr Brown was "beyond hope of redemption".
The fraught nature of the Blair-Brown relationship has long been known but Lord Mandelson’s memoirs reveal for the first time the level of animosity felt between the two.
Lord Mandelson disclosed that the then prime minister did promise the former chancellor in 2003 that he would step aside and not fight the next general election, a pledge he later reneged on.
At the same time, Mr Blair's advisers drew up plan "Operation Teddy Bear" to split the Treasury in two and weaken Mr Brown's power, it was claimed.