The Ministry of Health and Social Affairs confirmed the error, but did not specify the number of people affected.
An expert told Sky News he believes as many as four million people may have received incorrect updates by what he described as “a big finger mistake.”
The issue was noticed by some of the app’s 16 million users on Friday night, who took to Twitter to complain that the level of risk for their region had been changed in a way that contradicted official guidelines from the government.
Several users in Liverpool, the first city in the country to be placed in the strictest Level 3 restriction category for very high-risk areas, said their apps sent them an alert telling them falsely that they were demoted from very high risk to high.
App users in Sheffield, Nottingham and parts of the Midlands have reported receiving incorrect notifications that their risk alert level has been downgraded to medium.
“We live in Walsall and he is classified as high risk,” one wrote. “Why did we all get an alert saying our risk level has changed due to ‘increased risk levels’ when it is now MEDIUM? It is very confusing. ”
An expert who was monitoring the app at the time the error occurred said the issue was almost certainly caused by accidentally sending a blank file to the phones instead of updating the level of alert.
“I was monitoring changes to the app to see if they addressed any of the issues that were raised about confused risk level messages,” said Jeremy Place, an information security specialist.
“I noticed that the file was empty from 6:21 pm for about an hour. ”
A recent update to the app modified the postcode alert system to bring it into line with new government level classifications.
Previously, it had three alert levels: low, medium and high. The update changed them to medium, high, and very high.
Any phone receiving the empty file would have reverted to the old system, Mr Place said, generating an incorrect alert level and triggering a message saying, “The risk zone in your area has changed. ”
Several users reported the issue on social media by placing two phones with different alert levels next to each other.
The phone with the incorrect alert level displayed the old alert level format, confirming Mr. Place’s analysis.
Mr Place, who operates his own QR recording app, said the postcode alert level refreshed on phones every four hours, but believed that not all apps were downloading new files at the same time. time.
Normal practice, he said, would be to spread downloads to reduce pressure on the system, so that a quarter of apps were refreshed every hour.
Since the empty file was available for about an hour, about a quarter of the phones could have been affected by the error.
“We are told that 16 million phones have downloaded the contact tracing application. If those phones still use the app, it could affect four million people, ”Place said.
However, he added that only people who updated their apps to incorporate the latest changes would be affected.
When asked about the severity of the error, Mr. Place, an experienced systems architect, described it as “a big finger error” without “any technical risk”.
But he said, “What has been lost is the reputation aspect. You lose the reputation which is very important for this application. ”
In order to correct the error, the team behind the app sent another file at 7:31 p.m. As a result of this update, many other phones would receive a message that their alert level has changed, although the results of this second alert are correct.
News is the last challenge of the contact tracing app, which has been criticized for issuing false alerts telling people they were in the vicinity of someone who tested positive.
These messages, which are automatically sent by the Google and Apple system on which the app was built, caused widespread anxiety among users.
The Department of Health and Welfare said all false alarms should be ignored and isolation instructions will be clearly spelled out in the app.
Asked about incorrect risk level alerts, a DHSC spokesperson said, “We are aware of an issue that has impacted ZIP code alert updates for some users of the app.
“This was identified and resolved within an hour and users’ phones will automatically update to display the correct local alert level for their region with new instructions. “