Headteachers have warned they could still mount a legal challenge over GCSE English, as Ofqual said that this summer's results will not be re-graded.

The regulator's initial report found that January's GCSE English exams were "graded generously" but the June boundaries were properly set and candidates' work properly graded. Students who gained their English GCSE this summer will be given an extra chance to re-sit in November, Ofqual said.

But it added that it would not be "revisiting" the June grade boundaries because it would "contradict our responsibility to maintain standards over time and make sure results are comparable year-on-year." The June boundaries were right, Ofqual added.

The Association of School and College Leaders (ASCL) said it was not "acceptable or practicable" to make the students resit examinations. It warned it could still begin a legal challenge against grade boundary changes on the grounds that it had disadvantaged certain groups of students.

Headteachers had called for Ofqual to begin an investigation into this summer's GCSE English results after it emerged that grade boundaries had been moved during the year. As national GCSE results were published last week, angry headteachers claimed that exam boards had raised grade boundaries in English amid fears that too many children were going to get a C.

Shadow education secretary Stephen Twigg said that Ofqual's report was not good enough, and called on Education Secretary Michael Gove to make a statement to Parliament on Monday about how the government intends to address concerns about GCSE English.

ASCL general secretary Brian Lightman said: "We are actively considering what legal challenge would be a way of dealing with this, although we would much prefer to solve this through other means. What we are seeking to do here is get justice for young people who have been badly affected by systemic failures that are not of their making."

Ofqual chief regulator Glenys Stacey said the regulator looked carefully at how GCSE English qualifications had been awarded this year: "The issue is not the June but the January boundaries. Again, examiners used their best judgment in setting these boundaries, but they had less data and information to work with."

Ms Stacey said: "We have spoken to exam boards and they have been very responsive. Recognising the strength of feeling, they will be offering early resits for students who sat the June units. We will now go through our analysis and evidence with the representative groups for schools and colleges, so they can see it for themselves. We will also talk with schools, exam boards and assessment experts to see what lessons can be learnt and what can be done better in the future."

The National Association of Head Teachers (NAHT) said it was "extremely disappointed" in the findings of Ofqual's interim report. "The gravity of this situation cannot be overestimated," NAHT general secretary Russell Hobby said. "Our call for results to be re-graded remains. We will press for the report outcomes to be reconsidered and a re-grading to be carried out with the utmost urgency, to bring to an end the uncertainty over so many students' futures."