Reforms will stop schools using vocational qualifications to boost their GCSE scores, as the Government bids to end years of alleged game- playing condemned by education secretary Michael Gove as "morally wrong".
Under the proposal no qualification will be worth more than a single GCSE in the league tables.
The change removes at a stroke the multiple GCSE "equivalences" that act as the big incentive for schools to use vocational qualifications, which critics argue can be of little value to pupils.
It will be introduced - subject to a consultation this summer - in 2014 alongside a tough set of criteria that vocational qualifications will have to meet for league-table inclusion.
But heads' leaders are concerned that the measure is too "blunt" and will lead to schools abandoning worthwhile courses that would have benefited and motivated pupils.
The change comes as ministers revealed they would not add any further subjects to the English Baccalaureate, although AS-levels will now count towards the measure.
They have also reduced the minimum cohort for league-table inclusion from 10 to five pupils, leading to warnings that small schools will be unfairly penalised by arbitrary fluctuations in their results.
The new vocational qualification rules are proposed in response to the Wolf review of vocational education, which highlighted an "explosion" in the number of 14-16 "vocationally related courses" with GCSE league-table equivalences.
The number of such qualifications taken in schools rose from 1,882 in 200304 to 462,182 by 2010.
Professor Alison Wolf, the review's author, said this week that schools had "been under enormous pressure" to climb league tables.
"When any qualification under the English sun can contribute (to) these, the pernicious effects are obvious," she said.
In his foreword to the Wolf review, Mr Gove said it was "morally wrong" that pupils were taking courses of "little or no value" because of league- table incentives.
To be included in school league tables from 2014, vocational qualifications must have a "proven track record", offer progression to a broad range of qualifications, have a substantial proportion of external assessment and be graded rather than just marked as a pass or a fail.
No more than two non-GCSEs will be allowed to count towards the headline GCSE league-table benchmark.
But with many vocational qualifications worth four GCSEs in league tables and some as many as six, it is the plan to reduce their value to one GCSE that is likely to change school behaviour.
Anastasia de Waal, education director of think-tank Civitas, who has campaigned against the misuse of such courses, said: "Hopefully this move will do a lot to ensure that schools are entering vocational qualifications for the right reasons - that students are going to benefit."
But heads believe that although some GCSE equivalences may be overstated, the new rule is too crude.
Russell Hobby, general secretary of heads' union the NAHT, said: "Rather than applying an arbitrary rule, we should work out what the actual equivalence is according to the time it takes to teach these qualifications."
Association of School and College Leaders general secretary Brian Lightman said: "If the rule is as blunt as this, there is a real risk that worthwhile vocational courses will be squeezed out of the curriculum."
Exam boards have anticipated some of the post-Wolf changes. Edexcel was already planning to introduce external assessment to its BTECs which account for 13.5 per cent of all schools' league-table "GCSE" scores.
The most popular vocational qualification in schools is the OCR level 2 National in ICT, previously criticised as too easy by Ofqual and being of "doubtful value" by Ofsted.
Last week OCR revealed it was revising its entire range of Nationals to fit in with the Wolf recommendations. But even if the new versions from both boards do meet the new standard, the one GCSE equivalency rule is likely to reduce their take-up.
An OCR spokesperson said: "We believe that teachers choose those qualifications that best serve the needs of their learners. We will continue to prioritise the development of qualifications that meet those needs."
Rod Bristow, president of Edexcel's parent company Pearson UK, said: "We support any moves that clarify the true value of all qualifications to those studying them."
Original headline: Gove acts to end `game-playing' in league tables