Flaws in the government's Health and Social Care Bill could threaten the confidentiality of patient records, doctors' leaders have warned.
The British Medical Association says there are "serious concerns" about clauses in the bill on information sharing in a reshaped NHS.
The BMA says it fails to guarantee patients' identities are kept secret.
But the government says the bill makes no changes to any of the existing legal safeguards on confidentiality.
The BMA, which fears that patients might withhold important information because of confidentiality issues, has written to Minister of State Simon Burns.
The letter says the bill gives very broad powers to a number of bodies, including the Secretary of State, the Commissioning Board and the NHS Information Centre "to obtain and disclose confidential patient information for any number of unspecified health purposes".
"As currently drafted, there is very little in the Bill relating to confidentiality and information governance controls, which are so fundamental to medical practice and the trust-based relationship between doctors and patients," it says.
"In the course of consultation and treatment, patients will often disclose highly sensitive information to their doctors, information that can be vital to ensuring the optimal provision of appropriate care and treatment."
Dr Vivienne Nathanson, head of science and ethics at the BMA, accused the government of placing its desire for access to information over the need to respect patient confidentiality.
"There is very little reference to rules on patient confidentiality that would ensure patients are asked before their information is shared, or guarantee that the patient's identity will not be revealed.
"Fears that their data may be shared with others may result in patients withholding important information; this may not only affect their own health but has implications to the wider health service.
"By failing to put in place proper safeguards, the government is potentially removing the control doctors and, most importantly, patients have over their confidential data. This conflicts with government promises that patients will be given greater control over their medical records."
The BMA is proposing a number of amendments to the bill and their concerns are echoed by the Patients Association.
Chief executive Katherine Murphy said that very often patients had a very good relationship with their doctor and would disclose very sensitive information.
"Our concern is that patients won't know if that information was going to be shared and that may have a detrimental affect on the doctor/patient relationship."
A Department of Health spokesman said the government's modernisation plans would allow patients to see where unacceptable NHS services were being provided and should drive up the quality of care.
"However, there is no question of the Health and Social Care Bill undermining the confidentiality of patients and their clinicians. The bill does not change any of the existing legal safeguards, which are set out in the Data Protection Act and the common law of confidence.
"We are happy to work with the BMA to understand their concerns."