Exam marks in more than 10 per cent of A-levels, sat by tens of thousands of pupils, were "artificially" devalued in order to restrict numbers gaining the new A* grade, it has emerged.
The TES revealed the practice a fortnight ago, as heads feared it would mean some of the brightest pupils being unfairly downgraded and denied university places, which are already scarce.
At the time exam regulator Ofqual refused to say how often the controversial measure - which it agreed - had taken place, but insisted it would only have been used in "exceptional" circumstances.
Now the regulator's own figures show that the value of some marks was changed in 33 of the 214 summer A-level awards as part of an effort to keep the numbers of A* grades within expectations.
In 23 of those cases it led to the value of pupils' marks being reduced so that fewer candidates received A* grades than would otherwise have been the case. In the remaining ten cases, the value of marks was increased to boost the number of pupils getting A*s.
Andrew Grant, chair of The Headmasters' and Headmistresses' Conference (HMC), said: "I am concerned that this is now quite a high proportion of A-levels where this meddling has taken place.
"The fact that the adjustments have been concentrated in OCR - a board which traditionally has a high proportion of candidates from highly selective schools - only adds to our concerns."
A total of 83,078 candidates sat the 14 A-levels set by English boards where the value of marks was reduced. The OCR board set ten of the exams, and Edexcel and AQA two each. Other exams were set by boards in Wales and Northern Ireland.
The boards stressed that any concerned schools could make enquiries through the normal channels, but insisted that the A* grades awarded were "a fair representation of those candidates who achieved at the highest level".
Ofqual statistical guidelines, based on last year's results, suggested that 7 per cent of grades should be A*s, but the final proportion was 8.1 per cent. HMC feared the regulator had underestimated the extra motivation the new grade would provide.
Mr Grant said: "Yes, the proportion of A*s has been a bit above the modelling. The question (is), is it as far above the modelling as it should have been?"
Isabel Nisbet, Ofqual chief executive, said: "I am in no way complacent but I think it has been a very secure technical exercise."
Ofqual instructed exam boards to refer to statistical guidelines when deciding where grade boundaries should lie, so it could ensure that standards remained constant in a year of big changes.
This summer saw the number of A-level modules reduced from six to four, the introduction of the A* grade and of greater "stretch and challenge". The last change, which introduced harder questions, prompted newspaper accusations that the strategy had not worked because the pass rate and proportion of pupils gaining top grades rose again this year.
But experts have said that it was never designed to reduce passes or A grades, but to allow greater differentiation between the most able pupils, while ensuring that grades remained consistent with previous years.
A-levels where marks were devalued to restrict the number of A*grades, with overall numbers of entries
Classical Greek: 275
Religious studies: 9,146
Business studies: 4,531
Physical education: 9,997
Chemistry A: 14,931
Government and politics: 796
History of art: 908
Art and design: 4,709.