A-level students in England will be able to use grades in mock exams to progress to university and college courses and employment, following a last-minute U-turn in advance of results day on Thursday.
Results in mock tests - which were held before schools were forced to close amid the Covid-19 crisis - will carry the same weight as the calculated results to be awarded this month, Gavin Williamson said on Tuesday night.
Students who are worried their estimated grades are too low can opt to boost them by using mock scores if they are higher.
The move was described by one union leader as "panicked and chaotic," while Labour warned it did not go far enough to protect the "Covid generation".
This year's summer exams were cancelled because of the coronavirus pandemic and teachers were told to submit the grades they thought each student would have received if they had sat the papers.
Exam boards have moderated these grades to ensure this year's results - for students in England, Northern Ireland and Wales - are not significantly higher than previous years.
But now students in England awaiting their A-level and GCSE results can keep their grades in mock exams if they are higher than the calculated grade, with regulator Ofqual asked to determine how and when valid mock results can be used.
Students will have to go through the appeals process to use their mock exam result, with their school required to submit evidence to the exam board.
And they will still be able to sit exams in the autumn if they are unhappy with the grades they secured in mock exams, or if they are dissatisfied with results awarded by exam boards on Thursday.
All three grades will hold the same value with universities, colleges and employers, the Department for Education (DfE) said.
The Government has urged universities to be more flexible with their offers this year, while Kate Green, the shadow education secretary, called for offers to be more generous by law.
Mr Williamson said: "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."
Geoff Barton, General Secretary of the Association of School and College Leaders, said the plan created potential for "massive inconsistency" as mock exams were not standardised and some students may not have taken them before schools closed in March.
He said: "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'."
Lord Baker, who served as an education secretary under Margaret Thatcher, said it appeared Scotland had "got it right" and those that will suffer from downgrading are most likely to be the brightest students in the poorest schools.
He told the Telegraph: "Scotland makes it exceptionally difficult for the Prime Minister. It's egg on face, there is no question about that."
There are also fears mid-ranking A-level students could face a grading "lottery" after they were put in rank order against their classmates within each grade for each subject, according to Professor Alan Smithers, director of the Centre for Education and Employment Research at the University of Buckingham.
- Are you concerned about A-level results? Do you expect the new system to provide fair results? Share your thoughts on A-level exams being interrupted in the comments section below