Results of coronavirus tests carried out in an NHS hospital or Public Health England lab cannot be linked with the newly-launched NHS Covid-19 app, officials have admitted.
The app, which has been available for download across England and Wales since Thursday, has been hailed by Health Secretary Matt Hancock as an "important step forward" in the fight against the virus.
But officials said results from tests done in a Public Health England lab or NHS hospital, and those done as part of the Office for National Statistics' national surveillance programme, cannot currently be linked with the app.
The Department of Health and Social Care (DHSC) said it is aware of the issue and "urgently working" to resolve it.
The Welsh Government said there is no such issue in Wales.
In a tweet on Saturday after the DHSC statement, it said: "This issue doesn't apply to Wales. We took the decision to link our all-Wales laboratory testing systems with the NHS Covid-19 app.
"Here you'll get a code in your test result notifications to enter into the app, whether your result is positive or negative."
A tweet on the official app account on Friday said: "If your test took place in a Public Health England lab or NHS hospital, or as part of national surveillance testing conducted by the Office for National Statistics, test results cannot currently be linked with the app whether they're positive or negative."
The tweet came in response to a user who said they had a test and wanted to enter the result in the app but had been asked for a code, which they said they did not get with their test result.
Currently, the results of tests booked through the app are automatically recorded, a spokeswoman at DHSC said.
She said: "We are urgently working to enable positive tests for people who aren't already given a code to be added to the Covid-19 App.
"NHS Test and Trace will continue to contact people by text, email or phone if your test is positive, advising you to self-isolate and for those who don't have a code the contact tracers will shortly be able to provide codes to insert in the app.
"If you book your test via the app the results will be automatically recorded in the app and the isolation countdown will be updated."