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.
An A* could be obtained with only 47 per cent on one business studies paper, while 40 per cent earned an A on the paper taken by 8,605 pupils.
A hundred papers set by Britain's largest exam board, the AQA, required scores of only 45 per cent or less to gain a C grade.
The lowest mark required for a C was in a maths exam set by Edexcel, Britain's second largest board, which has been at the centre of a row over using administrative staff to mark papers.
The figures will increase the pressure on ministers over exam standards.
Last month, the Confederation of British Industry warned that many pupils leave school without 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. For a classical Greek paper, the figure needed for a C was 25 per cent, for a business studies paper, 27 per cent, for a physics paper, 28 per cent and for a science double GCSE, 31 per cent. In total, 100 of the 563 papers required a mark of 45 per cent or less for a C grade.
On an AQA business studies paper, the figures were even more striking for high grades - 49 marks out of 105 (47 per cent) merited an A*, with 40 per cent required for an A and 33 per cent for a B.
At Edexcel, the lowest boundary for a grade C was on 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.
This year, grade boundaries rose for Edexcel's maths GCSEs. For a C, the figure required for one of the papers was just 16 per cent.
On one of Edexcel's French papers, only 32 per cent was needed for a C. In German, the lowest figure was 36 per cent. For the popular double science course, pupils needed only 43 per cent for a grade C.
An Edexcel spokesman said: "Edexcel (is) confident that the grade boundaries set this year are commensurate with the boundaries from past years."
An AQA spokeswoman said: "We do not see the 45 per cent figure (to get a C grade) as anything unusual. Awarding is done on a thorough and robust basis."
On pupils being able to gain an A* with 47 per cent on the business studies paper, the spokeswoman said that grade boundaries had been lowered because pupils had found the exam "very difficult".
HOW TO GET A C GRADE
Maths: 20 per cent.
Classical Greek: 25.
Business studies: 27 Physics: 28.
Science (single award): 29.
Science (double award): 31.
Religious studies: 39.
Science (double award): 43.