QCA's Boston says diplomas are no quick fix
He is insisting that only those that can offer "industry-qualified" staff for the courses in schools and colleges, and top-class facilities, should be allowed to run them.
Mr Boston's commitment to the diploma reforms, however, was underlined as he signed an extension to his contract to remain at the Qualifications and Curriculum Authority until 2009.
Alan Johnson, the Education Secretary, has described the diplomas as the biggest education reforms in the world. From 2008, courses in five subjects - media, health and social care, construction, engineering and information and communications technology - will be offered in selected areas for 14 to 19-year-olds.
But speaking at a 14-to-19 education conference organised by the Specialist Schools and Academies Trust, held in London, Mr Boston said: "The diplomas carry great risks of failure. The greatest is if the first five diplomas were to be discredited by being offered in the wrong circumstances.
"They must only be taught by industry-qualified staff in good-quality facilities. There's a grave risk if, after 2008 and the first diplomas, we try to offer these qualifications everywhere to increase participation in post-16 education. My worry is if that happens, and we are building on a foundation of sand and not rock, by 2012 the bottom will fall out of these qualifications."
Past vocational developments, including GNVQs, are accepted by the QCA as examples of what can go wrong. Such courses, originally conceived as a mixture of academic and work-related learning, were widely criticised as becoming more like traditional exams because proper work-related teaching was hard to provide.
But Mr Boston said that the diplomas, if managed correctly, were the key to improving historically low rates of participation in post-16 education.
The QCA believes the new courses have a "wow factor" which will excite teenagers. They emphasise personal and thinking skills, as well as project work, to a degree that is not possible with GCSEs and A-levels.
Schools, colleges, local authorities and employers have until December to express interest in running the new courses jointly from 2008. The TES understands that all but two local authority areas have done so.
Bids will be whittled down and the first areas to offer the diplomas will be announced in the spring of next year. Mr Boston said that 50,000 teenagers could be taking them in 2008.
Some exam boards are concerned about the diplomas' tight development timetable. They must work out the details by May, though some key principles, including how the courses will be graded, have not yet been settled.