Pupils were able to gain a grade C on more than 100 GCSE papers this year with scores of 45 per cent or less, The TES can reveal. On one maths paper the figure was just 16 per cent.
On one business studies paper, for a course taken by 14,791 pupils, an A* could be obtained with only 47 per cent, while 40 per cent merited an A.
A hundred papers set by AQA, Britain's largest exam board, required scores of only 45 per cent or less to gain a C grade. Among them are some of the most popular exams in the country.
The lowest mark required for a C in a maths paper was set by Edexcel, Britain's second-largest board, which has been at the centre of a furore over the use of administrative staff to mark papers. Around a quarter of GCSE entries from Welsh schools are with English boards.
The figures, based on an analysis of grade boundaries at the two boards, will increase pressure on ministers over exam standards.
Last month, the Confederation of British Industry reiterated warnings that many pupils leave school without having mastered the basics, despite a steady rise in the numbers achieving top GCSE grades.
This summer, the AQA board set 563 GCSE papers. For two of the board's maths papers, only 20 per cent was required for a grade C. A quarter of the marks secured a grade C classical Greek GCSE, and 31 per cent the same grade at double science.
For a business studies paper, the figures were even more striking at the top end of the grade spectrum. Just 49 marks out of 105, or 47 per cent, merited an A*, with 40 per cent gaining an A and 33 per cent a B.
At Edexcel, the lowest boundary for a grade C was set for a maths paper.
Last year, The TES revealed how it was possible to gain a GCSE grade A in two Edexcel maths papers with a score of just 45 out of 100. Afterwards, the board wrote to schools to tell them that the paper would be made more "accessible" this year, implying grade boundaries could rise.
They did so for Edexcel's maths GCSEs, with pupils needing 59 per cent on one paper and 61 per cent on another, to gain a grade A. But for a C, one of the papers required just 16 per cent.
Meanwhile, GCSE and A-level coursework is being sold on the internet auction site eBay for as little as 99p. Teenagers are auctioning assignments in topics ranging from English literature essays on The Crucible to history tasks and science investigations.
Eight assignments were on offer this week. One student wanted pound;14.90 for a CD of a 38-page design and technology GCSE assignment, which the seller said earned "full marks 100 per cent" in 2004. Another item, with a starting bid of pound;1, on Shakespeare's The Winter's Tale, allegedly earned 44 marks out of 45.
Nearly 700 students were caught plagiarising by England's three exam boards last year, an 11 per cent increase on 2003. The eBay offers come after years of complaints about internet sites offering pupils access to thousands of coursework essays.