Plans to overhaul the exam appeals system are likely to harm students' future prospects, school leaders have warned.
The changes will make it harder for pupils to appeal against GCSE and A-level results, and fail to tackle the problem of ensuring that marks are accurate in the first place, according to two leading headteachers' groups.
But Ofqual, which has published proposals to reform appeals, said that the current system can be unfair and that changes are needed.
A consultation published by the exams regulator last year proposed that exam marks should only be changed when there had been a mistake in applying the mark scheme, or in counting the marks.
If there was a difference in opinion on how many marks a paper should be awarded - for example between the original examiner and someone looking at the paper when it was sent back for checking - then the original mark should stand if considered reasonable.
Other reforms include removing a Code of Practice which contains the existing rules for reviews of marking, moderation and appeals.
In response to Ofqual's consultation, the Headmasters' and Headmistresses' Conference - which represents leading private schools - and the National Association of Head Teachers (NAHT) said they were deeply disappointed by the planned reforms, and warned that they could be detrimental to pupils who rely on accurate marking to secure university and sixth-form places as well as jobs.
Chris King, Chair of HMC and Headmaster of Leicester Grammar School said: "These proposals are unfair, fundamentally flawed and likely to put even more pupils' life chances at risk. They must not be allowed to go ahead.
"We are deeply disappointed, as this was a major opportunity to make the system fairer and start to restore public confidence. We want reforms which help ensure exam marking is more consistent and accurate in the first place and rogue grades are dealt with transparently and expertly. Instead, these measures are likely to reduce the number of re-grades simply by making it harder to prove the original mark was wrong.
"The approach seems to be: 'we have too many complaints; let's make it harder to complain.' This is no way to restore confidence in fairness and accuracy."
NAHT general secretary Russell Hobby, said: "Students are entitled to a system that gets the marks right first time and is easy to challenge if something goes awry. Their futures depend on this, and their hard work demands it.
"Ofqual's test of 'reasonableness' is far too low a bar for marking accuracy. It effectively concedes that there are no objective grounds for preferring one mark over another, which rather calls into question the whole purpose of exams in the first place. The way to restore confidence in exam marking is to increase transparency and rigour - not to make appeals harder."
Julie Swan, Ofqual's acting executive director for general qualifications, said: "Our proposals were put together by talking to teachers, schools and others who have expressed concerns about the existing Enquiries about Results system, including HMC and NAHT. They were also informed by a significant piece of empirical research.
"The concept that students are either given a 'right mark' or a 'wrong mark' is a misunderstanding - often more than one mark can be a fair mark for a script. There is no question that marking mistakes should be avoided and corrected if they happen, but differences of professional judgement are a very different matter.
"The current system can lead some students to get a higher mark on review, even when the first mark was entirely consistent with the mark scheme. That is unfair to those who do not seek a review. Of course where the first result is not reasonable - and the mark scheme was applied incorrectly - these errors should be corrected and a new mark awarded."
Michael Turner, director general of the Joint Council for Qualifications, which represents exam boards, said: "It's disappointing that some organisations have decided to undermine the tens of thousands of teachers who each year mark exam papers to a world class standard.
"We have a rigorous and robust assessment system that requires these teacher-examiners to undergo thorough training and be continually monitored throughout the marking process. The regulator Ofqual's own research has shown it's a system to have confidence in.
"Where grades change, most are due to a legitimate difference in the two examiners' judgment and often found in subjects like English or history where there's a level of interpretation.
"It's a misunderstanding of the system to claim this equals poor marking and we believe the reforms set out by Ofqual will go a long way in recognising this. Of course, exam boards are committed to eradicating true errors and, in the small number of cases where they do occur, they're corrected."