Doctors have demanded an urgent review of a new NHS computer system over what they say is a lack of safeguards over sensitive patient information.
Some 85% of practices have access to choose and book
Nottinghamshire GPs found the new "choose and book" system could be accessed by NHS staff who were not based at an individual's practice.
Doctors told GP magazine it can include data on details such as hospital visits for abortions and mental illness.
Officials responsible for the system said they were looking into the claims.
Choose and book is being rolled out across GP practices in England to allow hospital bookings to be made online. It contains information such as address, date of birth, NHS number and history of hospital appointments.
It is not fully operational yet, with only 85% of practices having access to the system.
GP magazine reported that staff in practices where patients were not registered could bring up their details just by entering their name, sex - and a date of birth within 20 years of the correct one.
Dr Hamish Meldrum, chairman of the British Medical Association's GPs committee, said: "We've only heard about this potential problem in the last few days.
"We have written urgently to the person in charge of the choose and book system and have asked for an immediate investigation and response as to what the situation should be."
He added: "Obviously it is unsettling for GPs and particularly for patients if they feel the system is not secure in the way they were promised it would be."
A spokesman for NHS Connecting for Health, the government agency responsible for the IT upgrade, said the claims would be looked into, but also said there was a responsibility on NHS staff to act responsibly.
"As part of their terms and condition of employment, all NHS staff work within a code of conduct which governs the use of patient information.
"Everyone working for, or on behalf of, the NHS is responsible for the information they access, and has a legal duty to respect patient confidentiality and to keep information about patients secure."