Gordon Brown was branded a "disgruntled dour Scot" yesterday, who most voters see as uncaring.
The damning comments came in a book by Labour's former first minister, Henry McLeish.
Psychologist Anne Ellis also claimed that the Prime Minister's personality "just does not cut the mustard in popularity stakes", warning he may lose the next election to a "much more pleasant personality".
She made the remarks in a profile of Mr Brown contained in a new book by Mr McLeish looking at Scotland and Scots.
Scotland - A Suitable Case for Treatment, published on September 1, was written by the former first minister along with commentator Tom Brown, with a chapter contributed by Ms Ellis, a founder of the psychological profiling company PeopleMaps.
In it she suggested Mr Brown could learn a lesson from the current Scottish first minister Alex Salmond, who she describes as "a consummate politician if ever there was one".
Ms Ellis says Mr Brown is "a very typical example of a dour Scot" and adds: "He has no time nor interest in courting popularity as he wants to get on with solving the problems of the world."
She continues: "What he has failed to see is that in today's terms the role of politician, due to media attention, is more a public relations job."
Ms Ellis says the Prime Minister has "a very Scottish attitude of being doggedly determined and will probably insist on continuing to do the job his way".
However, she adds: "Right or wrong, elections are won these days on personalities and Gordon's just does not cut the mustard in popularity stakes."
And she says: "His being Scottish has been an issue with the UK electorate - this is not due to racism, but more to the fact of him being so typically representative of the disgruntled dour Scot, unwilling to give any quarter and play the popularity game."
The psychologist claims that the fact Mr Brown cares deeply about what he is doing does not come across, arguing "most of the electorate see him as uncaring".
In contrast she describes Mr Salmond, the SNP leader, as being "really gifted in telling people what they want to hear, always putting a positive spin even on bad news".
Ms Ellis claims the first minister shares this personality trait with a number of US presidents, including Bill Clinton. "It probably is the ideal personality for a politician to have, charming, charismatic and ruthless," she adds.
Ms Ellis goes on: "Alex Salmond could never solve the sorts of challenges that Gordon could, yet he can knock spots off him when playing to the gallery. Salmond knows that a politician needs to tell the electorate what they want to hear and then tell them again and again."
Speaking about the book in Edinburgh, Mr McLeish said: "What we tried to do was to start with the assumption Scotland is a country of great talent, great wealth and huge potential. But it is our basic premise that that is not yet being realised to the full."
He said he hoped the book would lead to a debate about Scotland's future.
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