A deputy head with a degree in maths who sat the functional skills maths test that forms a key part of the Diploma qualification has claimed some pupils would struggle to pass because it requires "too much stamina".
Other school leaders have also spoken out, warning that the functional skills tests could kill the Diploma "stone dead" because the maths element is too demanding.
John Downs, who is deputy head of specialist maths and computing at Conyers comprehensive in Stockton-on-Tees, said the questions on the level 2 test were too multi-layered and required "too much stamina and perseverance" for many students, even those who managed a good pass at GCSE.
Mr Downs said that although the maths involved was simple, the questions required the "maturity to work in a very careful and concentrated way on very boring and repetitive calculations".
So far, around 200,000 pupils have taken part in the functional skills pilot tests in maths, English and ICT, which form a core part of the Diplomas.
Mr Downs said: "Pupils are not used to that (boring and repetitive work) at GCSE and teachers tend to shy away (from it), so the question is how do we teach students to have that stamina and attention to detail?"
Twenty-two Year 9 pupils at Conyers are set to follow the creative and media Diploma. Mr Downs complained that the reading requirements in the NCFE board question paper were also quite high, and that there were ambiguities in the text.
Mr Downs said he felt pupils could be put off the qualification, which is worth more than four GCSEs, because it was like "putting all their eggs in one basket", with everything resting on the maths test.
Another head, Pat Wager, from Sacred Heart Roman Catholic High School in Newcastle, said that her pupils found the Diploma course "exciting and motivating", but it was "horribly unjust" that some of them would struggle to pass the maths elements.
Around ten out of 18 pupils on the level 2 creative and media Diploma had to retake the functional skills maths tests and were awaiting their results, she said.
"Sadly, several of them will shine on the principal and additional learning and will produce a super project but may not get the Diploma because they will struggle to achieve functional skills maths at level 2," Ms Wager said.
"There is no other qualification that requires a pass in maths to achieve. Ironically, some of them will achieve a pass in GCSE maths, but this will not count.
"This will kill the Diploma stone dead after this summer unless someone uses their common sense.
"It's a shame because the pupils are loving the course, finding it stimulating and well put together and it is delivering the promise in all other ways."
She said the school would return to BTEC qualifications if the papers or requirement for functional skills maths were not changed.
Exams boards have been piloting the Diploma functional skills tests since 2007, and are due to report on the outcomes shortly.
They have already admitted that the tests have been a major reason for more than a quarter of pupils on the first high-level Diplomas failing.
The confusion over functional skills is just one element of the highly complex Diploma programme that has led to widespread criticism.
Speaking at the annual conference of the Association of School and College Leaders (ASCL) last week, president John Morgan said the system needed to be simplified.
In his speech to the conference, Mr Morgan said the issues surrounding the functional skills maths tests were just one example of the "exasperation" among heads. He added: "There remain genuine concerns from colleagues who are frustrated at the current complexity of the diploma.
"We want the diplomas to succeed, but to do so, their structure must be simplified."
Meanwhile, a report published last week by exams watchdog Ofqual said the Diplomas were failing to stretch the brightest pupils.
The Conservatives have already indicated they would overhaul the system if they win the general election, and would scrap the three academic Diplomas promised in science, modern languages and the humanities.