Pupils in England will not be allowed to have their exam results upgraded when they are published on Thursday.
Education Secretary Gavin Williamson has ruled out England following Scotland in accepting scores estimated by teachers.
The Government announced late on Tuesday that A-level and GCSE students will be able to use results in valid mock exams to appeal if they are unhappy with their results.
He said: “We would have seen them shoot up, which would devalue the results for the class of 2020, and would clearly not be fair on the classes of 2019 and 2021.
“But worse than that, it would mean that students this year would lose out twice over, both in their education and their future prospects.”
Mr Williamson had earlier pledged the exams system will deliver “credible, strong results” for the overwhelming majority of young people, despite concerns that many could end up with results lower than they had expected to receive.
The Scottish Government on Tuesday confirmed that it would allow for all results that were downgraded to be withdrawn and replaced by the original estimates.
It followed protests from pupils across the country angry that they had been unfairly penalised by attending schools which have not historically had high levels of performance.
He said: “Pupils and parents are rightly worried that years of hard work are about to be undone because a computer has decided to mark their child down.
“For too long, the Tories have considered the needs of young people as an afterthought when their needs should have been central.
“It’s a blatant injustice that thousands of hardworking young people risk having their futures decided on the basis of their postcode.”