Business bodies representing employers who have drawn up two of the first five diplomas said a "risk assessment" should be carried out on whether the new qualification would be ready by next year.
The warnings, to MPs on the Commons education select committee, came with only 18 months to go before the first students were due to take the diplomas.
Exam boards have warned that the time they have to develop the new work-related courses - only five months - is the shortest on record for any major qualification.
The course specifications have to be sent to schools by the summer and there were concerns that there would not be enough sufficiently trained staff to teach them.
Some schools have also expressed confusion or a lack of interest in the diplomas, which are being offered in subjects including construction, health and social care, and engineering, from 2008.
Karen Price, chief executive of e-skills, which is developing a diploma in information technology, said: "I think we should delay a year if that is what's required." While John Rogers, chief executive of Skills for Health, which is developing the health and social care diploma, agreed that the Government had never properly assessed how well-prepared teachers were to start the new courses.
Ken Boston, Qualifications and Curriculum Authority chief executive, said a delay would cause "great disappointment and serious damage". But he also played down expectations about how many pupils would take diplomas in 2008.
The Government is keen on 50,000. But Dr Boston said this would only be possible if the regulator could guarantee all schools and colleges running the courses could do so to a high standard.