The NHS has removed health apps from its accredited library after researchers showed they were leaking people's medical details over the Internet.
The NHS Health Apps Library features apps that have been reviewed by the health service "to ensure they are clinically safe" and comply with the Data Protection Act, according to the NHS Choices website.
However, researchers from Imperial College London found several apps sent unencrypted personal and medical information over the internet, putting users at risk of identity theft and fraud and prompting the NHS to remove them from the library.
Lead researcher Kit Huckvale said: "It is known that apps available through general marketplaces had poor and variable privacy practices, for example, failing to disclose personal data collected and sent to a third party.
"However, it was assumed that accredited apps - those that had been badged as trustworthy by organisational programs such as the UK's NHS Health Apps Library - would be free of such issues.
He added: "Our study suggests that the privacy of users of accredited apps may have been unnecessarily put at risk and challenges claims of trustworthiness offered by the current national accreditation scheme being run through the NHS."
Health apps are used by an estimated half a billion people across the world and earlier this month Health Secretary Jeremy Hunt said his ambition was to get 15 per cent of NHS patients routinely reading and adding to their online medical records using smartphone apps within the next 12 months.
Apps registered on the service undergo an appraisal process that examines clinical safety and compliance with data protection law and to be listed developers are required to declare any data transmissions and register with the UK's data watchdog the Information Commissioner's Office.
An investigation of 79 apps listed on the service over a six month period in 2013, published in the journal BMC Medicine, found that 70 transmitted information to online services.
Some 23 of those sent personal information over the internet without encryption, while four were found to be sending both identifying and health information without encryption, allowing the researchers to use a hack known as a "man-in-the-middle attack" to capture the data.
"The results of the study provide an opportunity for action to address these concerns and minimise the risk of a future privacy breach" said Huckvale. "To help with this, we have already supplied our findings and data to the NHS Health Apps Library."
The researchers alerted officials in April and NHS Choices said the apps on the NHS Health Apps Library had been reviewed to make sure they were clinically safe and compliant with the Data Protection Act.
A spokeswoman said: "We were made aware of some issues with some of the featured apps and took action to either remove them or contact the developers to insist they were updated. A new, more thorough NHS endorsement model for apps has begun piloting this month."
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