Ofqual on Saturday set what constitutes a "valid" mock test for students who are appealing against A-level results in England.
But the regulator has now suspended these standards, and more information will be released "in due course".
One Conservative MP described it as an unacceptable "big mess".
A-level or GCSE students were unable to take public exams this year due to the coronavirus pandemic, and nearly 40% of A-level scores were lowered from teachers' expectations by the Ofqual algorithm.
Robert Halvon, Conservative chair of the Education Selection Committee, said the decision to review the appeal directives announced on Saturday only left students and schools in a state of confusion.
"This is a big mess. Good knows what's going on at Ofqual. It's the last thing we need at this time. It's unacceptable in my view," he said.
“Students and teachers are very anxious - especially students who are worried about their futures. This has to be settled.
"Ofqual shouldn't put things on websites, take them away, and create confusion. This is not available and needs to be changed."
Meanwhile, Ofqual's statistical model for grading faces two legal challenges that argue that students have been unfairly judged against the school they attend.
Ofqual said earlier on Saturday that in the absence of a written mock test, it would take into account other teachers' evaluations instead.
However, a statement posted late on Saturday evening on the regulator's website reads: “Earlier today we published information about the mock test results in the appeals.
"This policy is being reviewed by Ofqual's Board of Directors and more information will be published in due course."
“Bewildering, mind boggling, insufficient, shocked, shocked” - these are the more polite responses from school leaders, in an effort to understand Ofqual's peculiar retreat of its own rules on level 1 appeals.
An early morning email from a respected school principal was titled: "WTF?"
Ofqual is supposed to be an independent auditor for the exams, but assuming it hasn't canceled itself, who pulled the plug in what was announced in the fake exam appeals?
Failure to make a decision still leaves students waiting impatiently to see if they can appeal and claim their university places.
There is also the annoyance that the ministers did not manage this mess beforehand, or even when problems arose in Scotland.
Ultimately, whether it's by Ofqual, the Department of Education, or Number 10, a decision has to be made.
Do they adhere to and match current grades with an effective appeals system? Will this withstand the lack of selection of fair outcomes and legal challenges?
Or will they accept the political blow - and the risk of creating other types of injustice - by turning into teachers' expectations, as eventually happened in Scotland?
For Ministers, it is time to hand over the exam paper and begin their answers.
The BBC's political correspondent, Jonathan Blake, said the Education Department was apparently unaware of the change and a review of the standards was likely to spark further criticism of the government's handling of the examination process in England.
The criteria under which Ofqual accepts a "valid mock assessment," which is set on a Saturday and then suspended hours later, are set as follows:
- Supervised and invisible and conducted in conditions intended to secure employment as the student's work
- Either previous assessments issued by the relevant examination board, or assessments developed by teachers
- Taken under temporal conditions
- Completed before March 20, 2020, when schools and colleges were closed
- Marked using the score chart provided by the relevant test board
- Graded in line with the exam board exam standard
Before the results were announced, the Ministry of Education announced a "triple lock", meaning that students could accept the score calculated by Ofqual, appeal for a "valid fake result" or take the fall exams.
However, Labor said that under the new Ofqual standards, some students will not be able to use their fake results as the basis for an appeal.
Emma Hardy, Labor's shadow education minister and universities, told the BBC that the government had to "urgently approve" teachers ’grades as final results.
She said the UK government “failed to learn lessons from Scotland”, which has abandoned a similar attempt to adjust grades. "They are making it up and they are advancing and they simply have no evidence," she said.
Ms. Hardy also questioned whether Ofqual had the capacity to handle a large volume of appeals, saying, "This crisis has a ripple effect throughout the education system."
Legal challenges
Ofqual also faces two judicial review cases, as students challenge what they say is an "irrational and arbitrary" approach.
Student Curtis Parfit-Ford got his expected grades but told the BBC he wanted to challenge the "totally hilarious" system.