Gavin Williamson, the Education Secretary, is under pressure to follow suit in England ahead of A-level results day by doing away with the exam regulator's algorithm, which takes into account pupils' past performance as well as their school's historic grades.
On Tuesday, the Government tried to stave off criticism by introducing a "safety net" for A-level students in which they can use a mock exam as a basis for an appeal against their grade. The 11th hour move was criticised by headteachers, university lectures and vice-chancellors.
Professor Julia Buckingham, the president of Universities UK, which represents vice-chancellors, said: "This last-minute policy change presents a number of challenges for universities, and we are seeking urgent clarification from the Department for Education on a range of issues including the likely scale and timing of appeals."
Meanwhile, Geoff Barton, the general secretary of the Association of School and College Leaders, the largest union representing secondary heads, said the move "beggars belief".
"The Government doesn't appear to understand how mock exams work. They aren't a set of exams which all conform to the same standards," Mr Barton said. "The clue is in the name 'mock', and some students will not have taken them by the time that schools were closed in March. So this immediately creates the potential for massive inconsistency."
Mr Gibb said the statistical model being used to calculate students' A-level grades is "robust and fair", adding: "We are not apologetic for looking at every stage, even at the 11th hour, at systems we can introduce to make the system as fair as possible for young people."
He highlighted previous research by Ofqual which showed that, if teachers' predicted grades were allowed to be awarded unaltered, results would be 12 per cent higher than last year on average.
Dr Mary Bousted, the joint general secretary of the National Education Union, said: "Even these changes do not level the playing field between Scottish and English students.
"Scottish students were given the option of having the best grade awarded either by the exam board, or by teacher predictions. English pupils do not have this option. Their alternative to the exam board grade is the grades achieved in their mock exams, which do not take into account expected further progress.
"Most unfairly, English students are still more likely to be given lower grades, either by the exam board, or by the mock exam. This fundamental difference in the Scottish and English awarding process does not create a level playing field between Scottish and English students for university entrance."