None of the Government's Diplomas will survive in their current form if the Conservatives win the next election, The TES can reveal.
The party's education team believes the qualification is so flawed that it is considering abolishing it completely, despite the investment of pound;295 million that will have been made by March.
A Conservative administration could alternatively decide to reform the Diploma or cut it free from government, leaving exam boards to make a go of it if they chose.
But party insiders are clear that it cannot be allowed to continue as it is. They have canvassed opinion from across the education world, including civil servants, exam boards, subject associations and universities and concluded that there is little support for the qualification, designed to combine practical and academic learning.
The Tories have already said they will scrap the final three Diplomas in science, languages and the humanities, which are due to come on stream in 2011.
Martin Davies, deputy head of Ashfield School, Nottinghamshire - praised by the vocational education charity Edge for its pioneering curriculum - said he believed the Conservatives were correct to rethink the whole qualification.
"We have massive reservations about Diplomas," he said. The way they were being delivered in some schools, preventing pupils from taking wider options, was "tantamount to child abuse".
News of the Conservatives' lack of faith in the whole Diploma project follows ministers' high-profile launch this month of five new Diplomas in manufacturing and product design; business administration and finance; hair and beauty; environmental and land-based studies; and hospitality.
Ian Wright, schools minister, said research showed that "top universities overwhelmingly back the Diploma", which he called "a fantastic route into both higher education and employment".
But the government-funded research he referred to actually showed that all the most prestigious "research intensive" universities questioned lacked high levels of support for the Diploma, and were concerned about its academic rigour.
Last week another study revealed further scepticism, as well as ignorance, among teenagers about the qualification.
The findings have only increased Conservative misgivings. But the party has been reluctant to come out openly against all Diplomas for fear of being seen as hostile to vocational qualifications.
Insiders still feel bruised that their proposals to stop vocational qualifications being counted as A-levels and GCSEs in the league tables were portrayed as an attempt to turn the clock back to a two-tier system.
John Dunford, general secretary of the Association of School and College Leaders, said: "Schools and colleges are putting a huge amount of work into the development of Diplomas, believing them to be an important bridge qualification between the academic and the vocational, and would be very disappointed if the Conservatives scrapped them.
"But there is certainly room for the simplification of a complex structure."
Peter Mitchell, education director at Edge, backed the Diploma as a "good attempt" at linking academic and practical skills. While he hoped the Conservatives would not scrap it, he said it needed improving with more practical work, longer and more relevant work experience, and more employer assessment.
"I would urge them to review the Diploma and build on what is already there," he added.