Boris Johnson is facing calls to take charge of the growing A-levels “chaos” amid mounting anger among pupils, teachers and MPs.

There was disbelief at Westminster after guidance for children in England seeking to appeal against their grades being marked down was suddenly withdrawn without explanation.

One senior Conservative MP said the situation is a “huge mess” while Labour said it is up to the prime minister to “get a grip” and sort out the situation.

Labour leader Sir Keir Starmer condemned the government’s “chaos, confusion and incompetence”, adding: “Boris Johnson has been invisible during this crisis.

“He needs to take personal responsibility and fix it.”

Conservative former education secretary Lord Baker urged ministers to delay the publication of GCSE results due this week until the crisis over A-levels is resolved.

He said the statistical model used by the regulator Ofqual to standardise results across the country in the absence of actual exams was “flawed” and led to “hundreds of thousands of unfair and barely explicable downgrades”.

Some pupils gathered at Westminster to protest against the results, while others resorted to legal action in an attempt to get their downgrades reversed through the courts.

The latest row erupted after Ofqual issued guidance on Saturday setting out the criteria for pupils to make appeals on the basis of their mock exam results, only for it to be take down hours later.

In a brief statement, Ofqual said the policy is “being reviewed” by its board and further information will be released “in due course”.

Both Downing Street and the Department for Education declined to comment.

Conservative MP Robert Halfon, who chairs the Commons Education Committee, said the regulator’s actions have been “unacceptable”.

“That is a huge mess,” he told BBC News.

“Goodness knows what is going on at Ofqual. It is the last thing we need at this time. This is just unacceptable in my view.

“Students and teachers are incredibly anxious – particularly the students who are worried about their future. This has got to be sorted out.

“Ofqual shouldn’t put things on websites, take them away, sow confusion. This is just not on and it has got to be changed.”

The suspension of the Ofqual guidance followed claims by Labour that assurances given to pupils about the appeals process by education secretary Gavin Williamson are “unravelling”.

Williamson last week gave a “triple-lock” commitment that pupils could use the highest result out of their teacher’s predicted grade, their mock exam or sitting an actual exam in the autumn.

But the Ofqual guidance said if the mock result was higher than the teacher’s prediction, it was the teacher’s prediction that would count.

Labour deputy leader Angela Rayner said: “The Prime Minister must now take personal responsibility for this crisis by addressing the country in the next 24 hours to explain precisely how he will end this historic injustice.”

The latest setback comes as ministers are braced for a fresh backlash when GCSE results for England are announced on Thursday.

Like the A-level results, they will initially be based on teacher assessments and then “moderated” by the Ofqual algorithm to bring them in line with previous years’ results.

Williamson has said the process is necessary to prevent “grade inflation” that would render the results worthless after actual exams had to be abandoned due to the coronavirus outbreak.

Critics have complained it has led to thousands of individual injustices, disproportionately penalising those from schools serving disadvantaged communities.

Paul Whiteman, general secretary of the NAHT head teachers’ union, said all the 25,000 results that were two or more grades below their teachers’ predictions should now be subject to automatic review.

“It is quite clear that the rules are being written and rewritten on the hoof, and the people that are suffering are the thousands of young people who have seen their future options narrow and disappear through no fault of their own,” he said.

Geoff Barton, general secretary of the Association of School and College Leaders, urged the government to follow the example of Scotland – where there was similar outcry – and abandon the moderated results and go back to teacher assessments.

“It is time for ministers to stop the chaos and fall back on teacher-assessed grades rather than prolong this nightmare,” he said.