The NHS has removed several health apps from its library after researchers found they could put users' privacy at risk.
The move came after experts from Imperial College London alerted NHS officials in April to concerns about how data was handled.
Their study, on apps endorsed by the NHS Health Apps Library, found several sent unencrypted personal and medical information over the internet.
This puts users at risk of identity theft and fraud, they said.
They said the findings questioned the "trustworthiness" of NHS accreditation at a time when health apps covering a wide range of topics from weight loss to pregnancy were used by an estimated half a billion people across the world.
It comes after the Government announced that patients could soon access their medical records on a smartphone.
Published in the journal BMC Medicine, it also found 23 sent identifying details without protection, of which four apps sent both personal and medical information unencrypted.
The researchers used a form of hack known as a "man-in-the-middle attack" to capture the data sent by an app over the internet.
Lead researcher Kit Huckvale said: "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.
"The results of the study provide an opportunity for action to address these concerns, and minimise the risk of a future privacy breach."
Earlier this month Health Secretary Jeremy Hunt said his ambition was to get 15% of NHS patients routinely reading and adding to their online medical records using smartphones apps within the next 12 months.
A spokesman for the group medConfidential said: "This study proves what we've been telling NHS England for months.
"Quite a number of the apps they're currently endorsing are dodgy, and a significant proportion fail to meet even their own weak criteria."
NHS Choices said the apps on the NHS Health Apps Library had been reviewed following the concerns and were found to be 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."