People in England who test negative for Covid-19 are unable to share the result with the new NHS app for England and Wales if they did not book the test through it in the first place.
The app asks for a code to register a test result but a code is only received if the test is positive.
Those who enter that they have symptoms without entering a result find a self-isolation countdown begins.
There are also some positive results which cannot be uploaded.
Tests taken as a result of the Office for National Statistics surveys, and those taken in an NHS Hospital or Public Health England lab, cannot be shared on the app regardless of the result, according to a tweet from the app’s official account.
It is not clear how many test results would be affected by this.
The Department for Health and Social Care said it was “urgently” working on a fix for this and would be updating the app.
It also said that manual contact tracers would “shortly be able to” supply app codes when they contacted an individual who tested positive in this way.
People who have been using the app since its launch on Thursday, and who had already booked tests before downloading it, have found that they are unable to stop the self-isolation countdown after reporting symptoms if they then get a negative result, because it does not come with a code they can share.
How does the NHS Covid-19 app work?
“That’s so confusing as the app doesn’t tell you that can’t enter negative test booked outside it,” said Prof Deborah Ryan, who originally contacted the BBC.
“And the app still tells you to quarantine if you entered symptoms. So this means I can’t turn off the self-isolation alert in the app?”
The self-isolation alert cannot be de-activated in this situation.
In Wales, lab test results come with notification tokens for the app, Ifan Evans, health director for digital technology and transformation, tweeted.
The Department of Health said that using the app is “entirely voluntary” and advice to get a test or self-isolate cannot be enforced.
Tests booked via the app will have the results automatically shared with it, it said.
According to the data analyst App Annie the NHS Covid-19 app has been downloaded around 4m times so far.
“By downloading this app you are helping protect yourself and others. If you book your test via the app then the results will be automatically recorded in the app and the isolation countdown will be updated,” said a DHSC spokesman.
If you are asked to self-isolate for 14 days because you have been in close contact with somebody who has tested positive, you are advised by health officials to do so even if you later test negative yourself.