People can opt out of the system
Millions of patient records are to go online in London after long delays to an NHS IT upgrade in the city.
The £12bn government programme has been beset with problems and is four years behind schedule.
The records, which contain details of patient medications and allergies, will go live on Thursday following pilot studies across England.
It is hoped the system will allow data to be shared more easily. The scheme will also be rolled out across England.
The summary care record is designed to securely hold details of medications, allergies, adverse reactions and other key health information.
It is based on a patient's GP record but is designed so any doctor treating a patient can add to it.
It is hoped information will be more easily shared, for instance between hospitals and surgeries where a patient is treated.
The scheme should also allow doctors to rely on accurate information rather than patient recollection.
But in February, Andrew Way, chief executive of London's Royal Free Hospital, said a trial of the electronic records system had caused "heartache and hard work".
He said technical problems had cost the trust £10m and meant fewer patients could be seen, although he still supported the idea of putting patient records online.
The Department of Health has said lessons have been learned from that trial.
Critics have raised fears about data safety issues surrounding uploading potentially sensitive information.
Everyone across London will be given the chance to opt out of having a summary care record created.
Director of The Patients Association, Katherine Murphy, said the system had "great potential for making care safer".
But she added: "We have heard from patients who have found it is being made increasingly difficult to opt out. That is completely unacceptable."
Health minister Mike O'Brien said: "Having the right information at the right time can make all the difference to patients' experience of urgent care."
Ruth Carnall, chief executive of NHS London, said: "Getting hold of health records for London's highly mobile population often presents real challenges to doctors and nurses when patients need out-of-hours and emergency care."
Dr Grant Ingrams, from the British Medical Associtation said: "Electronic Summary Care Records have the potential to improve both quality and safety of patient care but it is critical for the programme's success that all patients receive balanced information and are made aware of their option to opt out."
The first London records are due to be uploaded in Southwark at the Princess Street Group Practice on Thursday.