Access to Cambridge is controlled by individual colleges
School-leavers will need to get at least an A* and two A grades in their A-levels from next year if they want to study at Cambridge University.
The change to the standard offer from AAA is subject to review but may be increased to more than one A* for science subjects within a few years.
Cambridge believes that, contrary to the popular view, the move favours state school over private pupils.
The A* grade will be awarded for the first time in 2010 for marks over 90%.
The proportion of A-level entries awarded an A grade last year was almost 26% on average across all subjects.
The more prestigious universities have been saying for some time that they find it hard to distinguish between the best candidates.
Last year Cambridge had so many applicants with at least three As that it had to reject more than 5,400 of them.
So as well as there being more stretching questions in the A-level exams, candidates who obtain the highest marks will be awarded an A* - as is already the case at GCSE level.
Last October the National Committee for Educational Excellence recommended that universities ignore the A* grade for the first few years after its introduction.
Its reasoning was that most offers are made on the basis of students' predicted grades, and there was a lack of reliable data on those.
The head of the Association of School and College Leaders, John Dunford, said: "We are disappointed that Cambridge has gone down this route, especially since other universities are likely to follow.
"With access to module grades and the new extended project, universities should have enough information, and information which is more precise than the A* grade, that allows them to discriminate between the best candidates.
"The inevitable consequence is that it will devalue A and B grades and increase stress and anxiety among bright 17 and 18-year-olds."
But the head of Cambridge's outreach working group, Churchill College senior tutor Richard Partington, told BBC News he did not feel it was premature to move straight away to using the new A*.
"The reason we are not jumping the gun is that we have been gathering marks from AS and A2, so we have been able to look at what would have happened had we been able to use A* last year," he said.
"It seems clear that it's perfectly safe."
Some Cambridge colleges have used students' actual marks from their A2 units - the second part of the A-level.
But Mr Partington said this was relatively complex as the marks were not issued by the admissions service, Ucas: they had to be obtained from applicants' schools.
The A* would be readily available, allowing the colleges to clarify and standardise their offer.
Challenge to others
Mr Partington said experience had not suggested that applicants from the independent sector were more likely to have the higher marks.
The effect of A*AA "looks neutral", he said.
"If we were to move perhaps in the sciences to using more than one A* there might be a widening participation benefit: there may be more state school students.
"But the syllabuses are also changing so we can't be sure."
He stressed that there would be a genuine review of the use of A* that could result in a reversion to the present system.
The Headmasters' and Headmistresses' Conference (HMC), representing many of the top independent schools, said it was delighted at the lead shown by Cambridge.
"Others will, without doubt, follow," said HMC secretary Geoff Lucas.
"As a minimum expectation, we challenge all universities to at least come clean and state, unequivocally, whether or not they, too, will use the A* in offers from this September.
"Students, whatever their background or schooling, deserve to know before September of this year."