Almost a third of GPs plan to opt their patients out of care.data if NHS England refuses to change the scheme to an opt-in basis, according to a survey by GP magazine Pulse.
The survey of 423 GPs found that 31 percent of respondents would opt their patients out of the controversial scheme despite it being against the law to do so.
Only 32 percent of GPs said that they would keep their patients opted into the scheme, while 37 percent of respondents replied 'don't know'.
The initiative, which aims to set up a database of people's GP medical records, was originally due to start extracting data from GPs in April.
However the start of collection was delayed in February to provide more time to inform the public, after a backlash from privacy experts who said that patients would be easily identifiable by data linkage.
In April NHS England's director for patients and information Tim Kelsey said that 100 to 500 GP practices will start to extract data this autumn 'to trial, test, evaluate and refine the collection process' before a national rollout.
The scheme is 'opt-out' so all UK citizens will be included unless they tell their GP otherwise. It is illegal for GPs to opt patient out of the scheme unless they receive instructions to do so from the individual concerned.
However many GPs are understood to be uneasy about the existing arrangement.
Delegates at the British Medical Association's (BMA) annual meeting in June voted to support an 'opt-in' model rather than the existing 'opt-out' arrangement preferred by NHS England.
GPs have expressed concern that patients are not sufficiently aware of the scheme and may confuse it with other schemes such as the summary care record, according to Pulse.
It added that many GPs who responded to the survey are worried that there are inadequate safeguards and a lack of clarity over data sharing rules.
In response to the findings, Eve Roodhouse, care.data's programme director, said: "Care.data aims to support entire communities, in line with the NHS's founding principle to serve everybody in our society.
"Data that underpins and reflects the needs of all of our communities is therefore needed to plan services most effectively and reduce health care inequalities, which is why the opt-out model is proposed by NHS England.
"However while it is important that people are properly informed about the aim of care.data, it is equally important that people know that they have the choice to opt out if they wish."