Business Secretary Lord Mandelson portrayed Gordon Brown as "insecure, self-conscious ... and angry", according to a leaked email.
The description was contained in a frank assessment of the Prime Minister's character, according to the Mail on Sunday which obtained the details.
It was sent in January 2008 - before Lord Mandelson rejoined the Government - to ex-Labour spin doctor Derek Draper who was then acting as an unofficial adviser to Mr Brown.
In it he states: "He is a self-conscious person, physically and emotionally. He is not as comfortable with his own skin as Tony (Blair) was (is). A new public persona cannot be glued on to him."
Lord Mandelson also complained that none of Mr Brown's advisers had spelled out his defects to him or told him how to put them right.
"The point you make about GB 'being himself' (whatever that is) is right. But this is not a substitute for policy formulation and taking well-prepared, well-ordered decisions. This is more the problem than telling people you watch X Factor," he said.
The disclosures came at the end of a week which has seen Lord Mandelson emerge as deputy prime minister in all but name after being credited with heading off a full-scale Cabinet revolt against Mr Brown.
A spokesman for Lord Mandelson told the Mail on Sunday: "This email was written over a year ago in response to the suggestion that Gordon Brown needed to change his image.
"What Peter was saying is that Gordon should just be himself, that artificiality doesn't work in politics and that what he is, what he stands for and the policies he is pursuing should be allowed to speak for themselves.
"He maintains this view."