The roll-out of a new NHS data-sharing scheme involving medical records should be delayed as patients have been left "in the dark", a patient watchdog says.
A giant database, Care.data, is being set up with anonymised records to help aid medical research and the monitoring of performance.
But Healthwatch England said the way NHS England had tried to explain the system had been confusing.
NHS England said privacy concerns were misplaced.
The central database - to be launched after April - will enable experts to assess diseases, examine new drugs on the market and identify infection outbreaks as well as monitor the care patients get.
Information is already available about what happens in hospitals, but to date it has been difficult to link those records with the information that was available about what is happening to patients when they are under the care of GPs.
However, concerns have been raised about the prospect of keeping all of the information in one place, with campaigners saying that it could lead to privacy problems and data breaches.
There is a proposal - to be discussed next month - which could give access to non-NHS bodies, including private firms.
Healthwatch England chair Anna Bradley said nearly a quarter of the 148 local Healthwatch groups had been in contact to raise concerns about the scheme in recent days.
It comes after NHS England started last month a mass mail out to every household explaining the project and giving people the chance to opt out.
Not everyone has received the leaflets yet, but Ms Bradley said there had been reports of people throwing away the leaflets as they did not realise what they were, struggling to understand what they were about and cases where GPs have decided to opt out all their patients themselves.
Ms Bradley said: "Once again the NHS has decided it knows best and is ploughing ahead with plans to share this data whether we like it or not.
"To make matters worse, the communications around this have been so poor that we are now in the situation where all of us are about to be automatically opted in to a scheme we know little or nothing about.
"While we recognise that sharing the data could be of significant benefit to researchers, the NHS has a moral duty to consult with all of us and trust us to make our own decisions."
She said Healthwatch England has now written a letter to NHS England chief executive Sir David Nicholson asking him to put back the roll-out of the scheme so patients could be consulted more.
Dawn Monaghan, of the Information Commissioner's office, also said she had concerns.
"At the moment, we don't think it is clear enough on the website or in the information that has been sent out exactly what data is going to go and what is not going to go."
But Tim Kelsey, NHS England's national director for patients and information, said patients need not have concerns as the information would not be "identifiable".
"This data is stripped of all the identifiers so, for example, the name and address features nowhere on this data; postcode, numbers, the data is stripped of all the identifiers and in their place are substituted meaningless pseudonyms in order that this data can be linked with other data sets.
"Can I be categorical? No one who uses this data will know who you are."