The most popular Diploma has been creative and media
The Diploma qualification, introduced this autumn, will need "significant adjustments" if it is going to gain popularity, says a college leader.
David Collins, president of the Association of Colleges, has raised concerns about the "operational complexities" of Diplomas.
Until the new qualifications were "proven", Dr Collins called for the existing ones to be maintained.
About 11,500 students have begun Diploma courses this autumn.
Speaking at the annual conference of the further education colleges' organisation, Dr Collins said that changes would be needed in Diplomas "sooner rather than later" if they were to become part of the mainstream and serve large numbers of students.
He also cautioned against damaging the established existing qualifications in the process of delivering the Diplomas.
"We support the new qualifications but there will need to be some significant adjustments if they are really to succeed on the scale that is envisaged.
"In the meantime, existing post-16 qualifications must be maintained until the new Diplomas have fully proven themselves.
"A skills element has to be introduced if they are to prove attractive to young people and the requirement for partnership working needs to change to give colleges the power and responsibility not only to take the lead in an area, but to deliver the new Diplomas independently if they so wish.
"There are some excellent examples of good practice, but as the offering scales up it is difficult to see how the operational complexities implicit in the present arrangements can be sustained. We must face up to that fact sooner rather than later.
"Most importantly of all we must not allow recognised weaknesses in the 14-16 school curriculum to drive out existing successful provision post-16."
A spokesman for the Department for Children, Schools and Families said its focus had always been on ensuring that Diplomas were high quality so it was rolling them out slowly and carefully - evaluating the programme as it is delivered over the next five years.
He added: "Schools and colleges need to work together and there is plenty of evidence they are already doing so - in fact 88% of colleges are already involved.
"There are examples up and down the country of schools, colleges, work-based learning providers, employers, and higher education striving in partnership to improve the local offer for young people."
Diplomas were introduced to offer a more attractive vocational qualification for teenagers, alongside academic Diplomas which could provide a route into higher education.
The first wave of the new qualifications were launched this autumn, with take up lower than had been projected.
Schools Secretary Ed Balls has told MPs the number of 14 to 19-year-olds starting courses last month was 11,490 - of which about 1,400 were taking Diplomas at a level equivalent to A-levels.
The most popular Diploma qualification so far has been creative and media. The other Diplomas starting this autumn have been information technology, health and social care, construction and the built environment and engineering.