England's exam watchdog is to begin an investigation comparing GCSEs and A-levels with the alternatives increasingly being offered by state schools.
The studies will look at both the IGCSEs and Pre-Us that ministers are keen to extend to the maintained sector and vocationally related qualifications such as BTECs and OCR Nationals that many state schools already use.
The news came as a TES analysis revealed that a surge in demand from schools for BTEC First qualifications means they contributed more in league-table values last year than all GCSE English and maths passes put together.
This week, think-tank Civitas claimed an educational apartheid was underway with lower-income pupils being "led away" from academic subjects towards "practically irrelevant" qualifications such as BTECs and OCR Nationals that were "mis-sold as 'vocational'".
Isabel Nisbet, chief executive of exam regulator Ofqual, said: "With increasing choice in qualifications there is a need to investigate comparability between them. One of the roles of the regulator is to help learners, employers and teachers understand the similarities and differences across the range of qualifications available in an open market so users can make more informed choices."
The watchdog will compare the level of demand from the different qualifications, their content and level of detail and whether the candidates' work is of a comparable standard across different qualifications.
However, it will not be allowed to investigate the point scores given to different qualifications in school league tables, which would require the consent of ministers.
Ms Nisbett told The TES that if schools use a wider variety of qualifications then it becomes more important for Ofqual to ensure there is clarity about what they deliver and how they help pupils progress.
Last year, teaching watch dog Ofsted warned that the OCR National level 2 in ICT and Edexcel's diploma in digital applications, which have seen huge growth in schools, were of "doubtful value". Critics believe schools opt for such qualifications because they are judged the equivalent of multiple GCSEs in the league tables despite needing less teaching time.
Ofqual will begin its comparative studies this autumn and report its findings next year.
The first will compare English literature and physics A-levels with the equivalent Pre-Us. A second study will compare GCSE, IGCSE, BTEC and OCR National science qualifications.
OCR said Civitas was "wrong" about OCR Nationals. They included practical and applied learning but the board had never claimed they fully prepared people for work, it said.
Pearson, parent company of Edexcel, the board offering BTECs, said the qualifications stretched and engaged pupils in a different way from GCSEs.
A Department for Education spokesman said: "We will reform league tables to ensure smarter accountability. Young people should be entered for the qualifications and a combination of subjects that are in their best interests - not to boost the league table position of the school."
Analysis, pages 14-15; FE Focus, page 31.