The comedian, who was recently pictured smiling with Labour's glitterati at 10 Downing Street, yesterday urged Mr Blair to give it a rest.
His outburst, in the forthcoming issue of the Radio Times, follows comments by several personalities, including Alan McGee, head of Creation Records, and Damon Albarn, of the Britpop band Blur, that the Government has failed to keep its promises.
Elton, until now regarded as one of the Government's staunchest supporters, said: "The most gruesome aspect of the Cool Britannia thing is the way the politicians are trying to latch on to it.
"It's sad, it really is. Leaders should never, ever try to look cool - that's for dictators. The obsession with cool is getting entirely out of hand."
In the article, entitled "Cool Britannia? What a load of Rubbish", Elton said: "The whole country is collapsing under a pile of labels. The Prime Minster has publicly stated that he wants to rebrand Britain. But the truth is that you can't buy cool and you can't create it with a label.
"And what, I should like to know, is so great about being cool anyway?"
He added that the celebration of cool was a destructive force and a source of trouble.
"Uncool people never hurt anybody - all they do is collect stamps, read science fiction books and stand on the end of railway platforms staring at the trains."
Then he rounded on the politicians themselves, accusing them of trying be cool to attract the voters.
"I don't mind Radio One trying to be trendy, but I can do without the Labour Party trying to strut its funky stuff. I did not vote Labour because they've heard of Oasis and nobody is going to vote Tory because William Hague has a baseball cap."
The comedian, famed for his attacks on Margaret Thatcher during the Eighties, warned the Government to watch its step.
"The present Government should be very careful: style is not substitute for substance.
Politicians have a tough job ... [and] should not make life even more difficult for themselves by committing the terrible mistake of trying to look cool."