A-level students in England are now being promised their final results will not be lower than their mock exams.
The Department for Education is announcing a “triple lock” - so results will be the highest out of their estimated grades, their mocks and a back-up written exam in the autumn.
It follows Scotland’s decision to switch to teachers’ predicted grades.
England’s Education Secretary Gavin Williamson said this would be a “safety net” for students.
But head teachers’ leader Geoff Barton was highly critical of this late change: “The idea of introducing at the eleventh hour a system in which mock exam results trump calculated grades 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. The clue is in the name ‘mock’,” said Mr Barton, leader of the ASCL head teachers’ union.
The heads’ leader said mock results were not consistent between schools and were not intended to be used in public exams and attacked the decision as “panicked and chaotic”.
After the cancellation of this year’s exams because of the coronavirus pandemic, students are being given replacement grades.
For A-level students in England getting their results on Thursday, mock exams marked by teachers before the lockdown will now become an important part of deciding their final results.
If the results students receive are lower than their grades in mock exams an appeal can be made to the exam board for them to be raised.
In Scotland protests have forced a u-turn over how grades are calculated - with 125,000 exam entries set to be upgraded.
Students challenged the fairness of linking estimated grades to the performance of a school in previous years.
It drew accusations that high-achieving pupils in low-performing schools would lose out - and that this would particularly discriminate against young people in deprived areas.
Instead students in Scotland will now get results based on their teachers’ predicted grades and without the moderating process that could lower these results.
But the link with previous school results remains an important part of how A-level grades will be decided in England, Wales and Northern Ireland.
The prior achievement of schools will be used alongside the ranking of pupils by schools.
Mr Williamson has rejected following Scotland’s switch to teachers’ grades.
And England’s exam regulator Oqual has warned that relying on teachers’ predictions would unfairly inflate results.
Using teachers’ predictions would have meant about 38% of entries would have been A* or A grades - considerably higher than the previous record of 27%.
Instead another measure is being introduced - based on the mock exams, taken in school and marked by teachers in preparation for the summer exams.
If pupils get an estimated grade lower than their mock exam this will be grounds for an appeal by their school.
This will be part of a so-called “triple lock” promise, so that students will get whichever is the highest grade from three types of assessment - their mock exams, the calculated grade and the written exams on offer in the autumn as a back-up.
“Every young person waiting for their results wants to know they have been treated fairly.
“By ensuring students have the safety net of their mock results, as well as the chance of sitting autumn exams, we are creating a triple lock process to ensure they can have the confidence to take the next step forward in work or education,” said the education secretary.
Labour leader Sir Keir Starmer warned that the approach to this year’s exams risks “robbing a generation of young people of their future”.
“It’s a blatant injustice that thousands of hard-working young people risk having their futures decided on the basis of their postcode,” said Sir Keir.
He called for a much clearer process for appeals and resits - and help for students most likely to be adversely affected by the system being used to calculate results.
The National Union of Students says that the rest of the UK should adopt the same approach as Scotland.
And Mary Bousted of the National Education Union warned it meant students in different parts of the UK were applying for the same university places with results based on “completely different criteria and wildly different pass rates”.
But it is expected to be a good year for students looking for university places - with an anticipated fall in overseas students leaving many places to be filled.
Even if pupils miss their required grades universities are likely to be “super flexible” this year, suggested Clare Marchant of the Ucas admissions service.
“Those near-miss candidates, if they’ve dropped one or two grades, universities are being super-flexible about that,” said Ms Marchant.