The news risks angering traditionalists, who will complain about the dumbing down of exams.
The new GCSE, being piloted in 80 schools this summer by the Oxford, Cambridge and RSA (OCR) board and likely to be introduced nationwide in 2006, involves all pupils taking two papers.
High-achievers, aiming for a B grade or above, sit two exams, one easier than the other.
On the tougher paper, a third of questions are set at A-star difficulty, a third are of A standard and a third of B grade difficulty. The board said that a pupil getting 75 per cent of the B grade questions correct would get a B regardless of how they did on the other questions.
With only a third of the questions on the paper of B standard, and three quarters of 33 per cent equating to 25 per cent, a candidate could achieve a grade B with a score of only 25 per cent on the whole paper.
Grade boundaries for this summer's exams have yet to be set, so the percentage figure may vary slightly.
Maths experts say the move, to be a part of the new two-tier GCSE which is being seen as the future of the qualification, stands to put off would-be A-level candidates by making them feel like failures.
Doug French of the Mathematics Association said he was concerned. He said:
"This is worrying. Giving a student a B on the basis of answering only a quarter of the questions right does not seem to me to be very sensible.
"The danger is that taking this higher paper will be a very dispiriting experience for candidates who might be thinking about doing A-level maths.
"If you end up answering only a quarter of the questions on the paper right, you are not likely to feel you are very good at the subject, even if you are told they were difficult questions and you have a B grade."
Last year there was controversy, acknowledged by the Government's maths inquiry, over pupils being able to achieve a C grade in the current GCSE, taken by most pupils, by scoring only 15 per cent on the top paper.
A board spokesman said questions in the new exam needed to be targeted at particular grade levels, and if a pupil answered a high proportion of the problems set at B-grade standard, it was right that they should be given a B.
The pilot was proving extremely successful in this, its second year, he said, with all schools involved in it last year continuing to put their pupils through the qualification.
Professor Adrian Smith praised the two-tier GCSE in February in his report on maths teaching. Last week, the Government, responding to Smith, proposed that the new structure be introduced nationally for first teaching from 2006, subject to trialling.
The report criticised the current three-tier structure of GCSE, which leaves around 30 per cent of pupils with no chance to achieve a C grade, as demotivating for thousands of youngsters. The new structure gives every pupil the chance to gain at least a C grade.
Tiered papers were introduced in subjects such as maths to stop hard questions turning off low-achieving teenagers, but not giving brighter pupils problems that are too easy.