Almost half of the public do not understand plans for sharing their medical records while 80% of GPs are unclear how the data will be used, surveys have shown.
Two polls for the Medical Protection Society (MPS) show most patients have not yet received leaflets explaining the system and GPs fear patients will not be informed enough to decide whether to opt out.
Last week, the Royal College of GPs (RCGP) warned of a "crisis of public confidence" in the new Care.data system. While it supports the plan in principle, it said it was "very worried" the public had not been properly informed.
There has been huge criticism of Care.data, which is being established to aid medical research and monitor NHS performance more closely.
The idea is to link data from GP records with information from hospitals to give an idea of what happens to patients along the way.
The data being extracted from GP systems includes things such as family history, vaccinations, referrals for treatment, diagnoses and information about prescriptions.
It can also include biological values such as a patient's blood pressure, body mass index and cholesterol levels.
Personal confidential data (PCD) identifiers will also be included such as date of birth, postcode, NHS number and gender. The written notes a GP makes during a consultation will not be included in the information, which will then be anonymised and held centrally by the NHS Health and Social Care Information Centre (HSCIC).
Today's YouGov survey of more than 1,400 members of the public showed 67% have not received the leaflet from NHS England explaining the new system.
Furthermore, 45% do not understand Care.data from what they have read or heard.
However, half of those surveyed said they were not concerned about their data being shared outside their GP practice.
A separate MPS survey of more than 600 GP members showed 77% do not think NHS England has given them enough information to properly inform patients about Care.data.
Some 80% of GPs said they themselves do not have a good understanding of how the patient data will be used while 55% had concerns about the overall system.
Of those GPs that had concerns, 67% think patients will not have enough information to make an informed decision on what happens to their records.
However, 28% believe the system will join up medical records while 32% think it will help inform decisions about improving care in local communities.
Nevertheless, 80% said the system could undermine public confidence in the principle of medical confidentiality.
NHS England has given assurances that insurance companies will not be sold data for insurance purposes. But the plans mean that private health companies that happen to have an insurance arm could be allowed access.
There is also a proposal, being discussed next month, which could give non-NHS bodies, including pharmaceutical companies, access to the data.
Dr Pallavi Bradshaw, medico-legal adviser at MPS, said: "While we recognise that sharing information about patients could transform the way the NHS cares for and treats people, it is worrying that GPs feel that there is a lack of information for patients to make an informed decision about their personal data.
"This is a huge step in modernising health services, which most people will only find out about in a mail-drop to households, and that may get lost or discarded along with take-away menus and supermarket offers."
Dr Bradshaw said technology could offer enormous opportunities "but we do not want this to be at the cost of trust between the doctor and patient".
He added: "Although the results tell us that half of patients are not concerned about their medical records leaving the GP practice, we worry that this is because, historically, patients have had confidence in their GP to look after their personal data.
"Some patients may see the scheme as an unwelcome intrusion into their personal lives which could irreversibly damage the relationship with their family doctor."
A spokeswoman for NHS England said: "We are absolutely committed to ensuring the public understand the benefits of this important initiative and also the choices available to them.
"This is why we provided leaflets and posters to every GP practice in August 2013, have produced a video animation, and have established an information line - 0300 456 3531 - mfor patients to call if they have any questions or concerns. Information is also available on NHS choices.
"We contracted Royal Mail to deliver a leaflet to every possible household in England during January.
"We are concerned by reports that some households have not received a leaflet and are following this up with Royal Mail as a matter of urgency."