MPs are warning the government it risks failure if it rushes in new Diplomas for teenagers which are aimed at mixing practical and theoretical learning.
The Diplomas will come in from 2008 in England
The Education and Skills Committee complains of confusion over the aim of the changes and says ministers might need to delay the full implementation.
From 2008, teenagers in some parts of England will be able to study for Diplomas in five work-related areas.
Ministers say the Diplomas represent the biggest change in a generation.
The first five Diplomas are in creative and media; construction; engineering; health and social care; and IT.
The plan is for all students to have the option of studying a wider range of Diplomas, 14 in total, from 2013.
The qualifications will be available at various levels - from the equivalent of four or five GCSEs up to that of three A-levels.
MPs on the Education and Skills Committee complain of confusion among those developing them.
"It is far from clear those in charge of developing the different Diplomas share a common understanding of the kinds of learning they will demand and the purposes they will serve," the report says.
It also says that at times there was a lack of clarity about who was taking key decisions on the content and design of the Diplomas.
The MPs warn of a muddle over the range of qualifications, saying it is not clear how existing qualifications - both vocational and academic, as they are commonly categorised - relate to Diplomas.
The government says GCSEs, A-levels and vocational qualifications will remain and that A-levels and GCSEs could be included within the Diplomas.
Pupils taking Diplomas will also have to pass tests in maths and English. They will have compulsory work experience and will spend 50% of their time in work-related learning.
The MPs say the government should treat the first year of the Diplomas as a genuine pilot, and as an opportunity to remedy any problems which crop up.
And it should be ready to delay the expansion of the programme if necessary.
"The government needs to ensure that it is genuinely a pilot, and if problems are not resolved, or if further problems emerge, then the wider roll out should be delayed or reviewed in order to prevent the failure of the Diplomas," the report says.
Education Secretary Alan Johnson said the Diplomas would deliver "essential skills and knowledge, hands-on experience and employer-based learning, to prepare young people for work or further study".
"They are a radical transformation of the choices available to young people. They are a genuinely exciting change to our education system," he said.
MP Barry Sheerman, chairman of the committee, said: "The government must ensure the Diplomas are a success from their inception.
"These Diplomas have the potential to offer a unique and valuable pathway to skills and qualification for young people. This is an opportunity too precious to miss."
The government says it will respond to the select committee report in detail in due course.