The new top A-level grade, the A*, which will mark out the highest flyers, will require marks of at least 90%.
The top A-level grade will rise from 80% to 90%
An increasing number of A grades are being awarded each year.
The A* is intended to highlight the best candidates, without altering the standards required for the existing five A-level grades.
The government agrees with the QCA exams watchdog the grade boundary should be 90%. Head teachers said it could make an A grade "second best".
The Qualifications and Curriculum Authority (QCA) had recommended to the outgoing Education Secretary, Alan Johnson, that this new top-level grade should be reserved for students who had an aggregate score of at least 90% across the units of their A-level.
He has accepted that.
He wrote: "In November when I agreed to your proposal to introduce an A* grade into A-levels, I was very clear that the way the A* is determined should be simple and transparent and accessible to all of the most able students, wherever they receive their education.
"I am pleased that the modelling of your proposal meets these conditions."
There have been calls for this extra level in response to the increasing proportion of students getting an A grade - which has now risen to almost a quarter of entries.
With an increasing number of students getting A grades in all their subjects, this has made the selection process more difficult for university admissions officers.
Among pupils at independent schools, almost half of all A-level entries achieve an A grade.
The existing A grade is awarded to students scoring 80%, and the new higher level is intended to identify the most talented students within this group, without changing the standards required for the five existing grades (A to E).
The former education secretary, who wrote to the QCA in April, wants the A* to be tested in "modelling exercises in 2007 and 2008" to make sure that "the new assessments maintain the A-level standard".
The A* has been scheduled to be fully in place from 2010.
The introduction of a higher level was welcomed by the Conservatives as a "long overdue" recognition that there had been grade inflation at A-level.
"This announcement is an important first step in restoring integrity and rigour to the A-level exam," said the Conservatives' schools spokesman, Nick Gibb.
But John Dunford, general secretary of the Association of School and College Leaders, opposed the higher grade.
"I regret the introduction of A* since it has the potential to make a grade A second best," said Dr Dunford.