David Cameron lacks the "slightest sense of purpose" and is surrounded by a band of mediocre sycophants, according to one of Michael Gove's closest allies.
In a blistering attack, Dominic Cummings accused the Prime Minister of "bumbling" and described him as being "a sphinx without a riddle".
The Education Secretary's former adviser claimed the senior team around Mr Cameron were "far out of their depth" and have "no grip".
In an interview with The Times, he said : "As Bismarck said about Napoleon III, Cameron is a sphinx without a riddle. Everyone is trying to find the secret of David Cameron but he is exactly what he appears to be.
"There's no mystery to him. He had a picture of Macmillan on his wall - that's all you need to know."
Mr Cummings, who quit as Mr Gove's adviser earlier this year, is known for his robust outbursts, including accusing Deputy Prime Minister Nick Clegg last month of lying over how plans to introduce free school meals for infants would be funded.
He dismissed Ed Llewellyn, the Prime Minister's chief of staff, as a "classic third-rate suck-up-kick-down sycophant presiding over a shambolic court".
"To get anything done you have to have priorities and there are no priorities. Everyone is discouraged from telling the truth to important people," he said.
"There's no grip, no focus. I thought it was a shame.
"Llewellyn tried to stop an awful lot. Pretty much everything.
"I think No 10 should have made education a fundamental priority."
Mr Cummings also claimed the civil servants who have remained in the Department for Education are now loyal to Mr Gove and support him in trying to block moves by the Liberal Democrats and other departments.
"They now work with him to screw other people in Whitehall. So if (Nick) Clegg comes along with another mad idea, our officials work with Michael to scupper it."
A source close to Mr Gove said: "Michael didn't know Dominic was doing the interview and he disagrees with his comments on No 10."
Mr Cameron's official spokesman sought to shrug off the attack, telling a regular Westminster media briefing that the Prime Minister "doesn't remotely share your interest" in Mr Cummings' remarks.
"These things crop up in the papers," said the Prime Minister's spokesman. "The Prime Minister is focused on the Government's long-term economic plan and the reforms he is taking forward."