Patients risk having less of a say in the running of the NHS in England under the proposed shake-up, eight leading health charities say.
GPs are being given control of much of the NHS budget under the changes laid before Parliament last month.
Part of the reasoning was that it would help empower patients, but the groups, including the British Heart Foundation, said it could weaken their involvement.
The government insisted the changes would give patients "real clout".
The intervention by the health charities - in a letter published in the Times - follows widespread criticism of the reforms by health unions and MPs last month when the bill paving the way for the changes was published.
Managers working for primary care trusts (PCTs) are currently responsible for planning and buying local services, but GPs working together in consortia would take on responsibility for this from 2013 under the reforms.
Pilots are already starting and once the process is complete, two tiers of management - PCTs and the 10 regional health authorities - will be scrapped.
The letter, which was also signed by the Alzheimer's Society and mental health charity Rethink, questioned the powers and resources being given to the local Health Watch bodies, which will be set up across the country to represent patient interests in the new NHS structure.
It states: "If the new NHS is to properly serve patients and the public, this democratic deficit must be addressed and the voices of patients heard by those making crucial decisions affecting their lives."
The letter also said the proposed scrutiny arrangements - local authorities are being put in charge of monitoring GP consortia - meant the lines of accountability were "too weak".
Andrew Chidgey, of the Alzheimer's Society, told the BBC the issues needed addressing now that the bill was working its way through Parliament.
"If we don't do this well and thoroughly we could end up with patient and public involvement weaker than it is and no-one really wants that."
The eight signatories to the letter are the Alzheimer's Society, Asthma UK, Breakthrough Breast Cancer, Diabetes UK, National Voices, Rethink, the British Hearth Foundation and the Stroke Association.
Their criticisms represent a new area of concern about the changes.
A Department of Health spokeswoman said the letter raised "constructive" points, adding: "We will work together to ensure the bill, which is in its early stages, delivers the reality of improved patient involvement."
However, she disputed the suggestion that patient power could be diluted, citing the ability of Health Watch to trigger official NHS inspections.
Last month, the House of Commons' health committee said the plans had taken the NHS by surprise and could threaten its ability to make savings.
Meanwhile, in another letter, also published in the Times, six health unions, including the British Medical Association, warned of their "extreme concerns" that greater commercial competition in the NHS would end up undermining care.
The NHS Confederation, which represents managers, has also suggested hospitals may have to close.