Trials of the NHS's controversial electronic patient record system will start in the spring — but patients will be allowed to opt-out of data sharing if they want to.
The electronic record will contain information such as patients' current medications, allergies and adverse reactions.
Because the information will be available to medics anywhere in the country, ministers argue the system will help patients by giving A&E doctors or paramedics access to vital information that would otherwise sit in a GP's files.
But the project has faced stiff opposition from patients who are concerned that their personal records will be at risk from snoopers or hackers.
The government has now announced that in the trial, patients will be able to see their proposed NHS Summary Care Record on the HealthSpace website or by requesting a printed copy. They can correct or amend their record and either consent to sharing it or opt out.
But Ross Anderson, professor of security engineering at the Cambridge University Computer Lab, warned the system is still potentially "unsafe" because personal information will be made available to a large number of people. And because the system is centralised there will be a risk of harm when it fails, he warned.
He told BBC Radio 4's Today programme: "Once your information has been uploaded from your GP to the central systems you will have an enormous job in getting it deleted or getting it changed if it's wrong and you will have no control over who outside of the health service is able to use it, so it's far better to opt out now."
But health minister Lord Warner insisted that access to the system — by smartcard and PIN — would be secure and warned: "We have to make sure people understand the risks they are taking by not allowing this information to be shared."
The chairman of the London Ambulance Service NHS Trust Sigurd Reinton warned that the elderly and vulnerable should bear in mind that if paramedics have more information they will be able to offer the "best possible treatment".