Hospitals across England are using 21 separate electronic systems to record patient health care - risking patient safety, researchers suggest.
A team at Imperial College say the systems cannot "talk" to each other, making cross-referencing difficult and potentially leading to "errors".
Of 121 million patient interactions, there were 11 million where information from a previous visit was inaccessible.
The NHS said it was working to ensure different systems could work together.
The electronic medical records (EMRs) system was launched in 2002 with the aim of allowing clinicians easy access to all the information on a patient, even if they had previously been treated elsewhere.
But it has been plagued with delays and operational problems ever since.
The team from London's Imperial College's Institute of Global Health Innovation (IGHI) looked at data from 152 acute hospital trusts in England, focusing on the use of EMRs on the ward.
Around a quarter were still using paper records.
Half of trusts using EMRs were using one of three systems: researchers say at least these three should be able to share information.
Ten per cent were using multiple systems within the same hospital.
Writing in the journal BMJ Open, the researchers say: "We have shown that millions of patients transition between different acute NHS hospitals each year.
"These hospitals use several different health record systems and there is minimal coordination of health record systems between the hospitals that most commonly share the care of patients."
Dr Leigh Warren, who worked on the research, said: "Patients expect their health records to be shared seamlessly between hospitals and healthcare settings that they move between.
"They cannot understand why, in the NHS, this is not the case."
"Yet hospitals and GPs often don't have the right information about the right patient in the right place at the right time.
"This can lead to errors and accidents that can threaten patients' lives."
Lord Ara Darzi, lead author and co-director of the IGHI, said: "It is vital that policy-makers act with urgency to unify fragmented systems and promote better data-sharing in areas where it is needed most - or risk the safety of patients."
A spokesperson for NHSX, which looks after digital services in the NHS, said: "NHSX is setting standards, so hospital and general practioner IT systems talk to each other and quickly share information, like X-ray results, to improve patient care.