A laptop containing 8.6 million medical records has gone missing from an NHS building in London, according to a report.
The computer was lost three weeks ago but police were only informed this week, according to The Sun.
The laptop was reportedly unencrypted and contained sensitive details on 8.63 million people. It also held records of 18 million hospital visits, operations and procedures.
The Information Commissioner's Office (ICO) confirmed it is looking into the issue.
"Any allegation that sensitive personal information has been compromised is concerning and we will now make enquiries to establish the full facts of this alleged data breach," an ICO spokesperson told IT Pro.
Data lost did not include names, but gender, age and ethnic details were held on the laptop. Other data included details of cancer, HIV, mental illness and abortions.
The device was one of 20 that went missing from a store room at NHS North Central London. Eight have been recovered, with searches for the other 12 ongoing.
"When a machine contains highly sensitive information on literally millions of patients, not securing the data on it by any means possible isn't just careless: it's sheer negligence," said Chris McIntosh of ViaSat UK.
"The ICO has proven several times that it is willing to impose civil penalties on public sector organisations. It is to be hoped that the ICO acts swiftly and decisively to pass a strong message in this case and that, more importantly, the data on the laptop itself doesn't end up in the wrong hands."
Nick Lowe, head of sales in Western Europe for security firm Check Point, said the new case again highlighted the need for organisations to better secure their hardware.
"The scale of this potential data loss drives home just how essential it is to have mandatory, strong encryption on all sensitive, personal on laptops and portable storage devices even if those devices are stored in supposedly secure areas within buildings," Lowe said.
"Data security is still being mostly left to chance."
The NHS has come under the data security spotlight numerous times in recent years, thanks to repeated device losses.
