The NHS Confederation has called for action to reduce risks associated with the design and implementation of the government's healthcare reforms.
The Confederation said its members support the government's objectives of empowering patients and involving clinicians more closely in decision making.
But it also indentifies "significant risks, worrying uncertainties and unexploited opportunities" that need addressing if the plans are to work as intended.
It has published its response to the white paper Equity and Excellence - Liberating the NHS on behalf of 95% of organisations in all parts of the NHS, the result of a consultation exercise.
Nigel Edwards, acting chief executive of the Confederation, said: "It is imperative that the government does everything possible to address what is a significant list of uncertainties about how the new system will work.
"The fact of the matter is that the government is planning to build a very big new machine - at great pace - but no one can be quite sure what will happen when it is switched on."
The Confederation makes 40 suggestions for improving the new system and proposes a ten-point action plan for managing the transition.
Key concerns about the proposed new system include:
GP consortia do not appear to be clearly accountable to patients and the public, GPs need to be fully engaged in commissioning, and accountable to the populations they serve.
Market mechanisms alone will not be adequate to manage the system There needs to be a way to encourage GP consortia to work together and help providers adjust to changes in demand.
There is a need to promote better integration between primary and secondary care; and between health, social care and public health. But some of the incentives and structures in the proposed system could act against this.
The proposals do not make the most of the potential of GP consortia to drive health improvements. Giving consortia the power to performance manage primary care and make the tough decisions needed to get the best value for public money could make a big difference in these areas.
Edwards added: "Ministers also need to ease the very deep worries that people feel about the transition. This is the area where people's concerns have been greatest because there is a real danger of failures in quality of care or finances. We are about to embark on a hazardous journey at a time when resources are hugely stretched. The risks are very real indeed."
