New exams taken by 16-year-olds in England reveal thousands struggled to grasp basic English and maths, including grammar, punctuation, percentages and fractions.
From 2010 all pupils will take a "functional skills" paper as part of GCSEs in the two subjects.
Tests are designed to show that school-leavers are competent in the foundations of literacy and numeracy.
But trials of the exams - carried out three times last year - show the majority of young people failed.
Critics said it provided a more accurate picture of standards achieved by teenagers in England because - unlike in existing GCSEs and Sats tests - they had not been "coached" to pass beforehand.
Documents published under the Freedom of Information Act show only 30 per cent of candidates passed the first maths test, which was run by the exam board OCR in January.
This dipped to 24 per cent in a test in March, then rose slightly in June to 35 per cent. The pass rates in the three English papers was 66, 41 and 57 per cent.
It suggests pupils are failing to learn key concepts such as spelling and commas in English and measuring distances and graph reading in maths. These are topics that critics say should have been mastered in primary school.
Low pass rates will also send GCSE results plummeting when functional tests become part of the GCSE requirement from 2010.
Unless pupils pass the functional skills papers, they will not be able to gain a grade C or above in GCSE.
Alan Smithers, professor of education at Buckingham University, said: "It is a comment on the system that after 11 years of formal education in English and maths pupils can not carry out basic operations successfully. Pupils may not have received specific teaching in the tests but if they are studying maths GCSE, they should be able to tackle this material."
In traditional GCSEs last year, 63 per cent of candidates achieved at least a C grade in English and 56 per cent in maths.
Jim Knight, the Schools Minister, said: "It's normal for exam results to be erratic for pilot qualifications, and this was the first set of results from the first set of functional skills pilots. We expect these results to settle as teachers and students get used to the new use of their skills.” (By Graeme Paton, Education Editor Last Updated: 6:36PM GMT 27 Feb 2009)