Around 4,000 people in Wales may have been given a false negative Covid PCR result as testing is suspended at a private laboratory.
NHS Test and Trace has launched an investigation at the laboratory in Wolverhampton, which is operated by Immensa Health Clinic Ltd.
Up to 43,000 people across England and Wales could have been given false negative results - with the majority of people affected being in the South West of England.
Test results affected were issued between 8 September and 12 October.
Health Minister Eluned Morgan MS said the "majority" of the 4,000 tests will have been taken at testing sites across the Gwent and Cwm Taf Morgannwg areas.
She said the "issue is isolated to this laboratory and does not involve the wider network including the IP5 laboratory in Newport that processes the majority of Welsh samples."
"Anyone who had a test from 4 October and received a result from the affected laboratory will be contacted by NHS Test and Trace by text message and/ or e-mail and advised if it was negative to book an appointment to be re-tested."
"It will also advise that their close contacts who are symptomatic book a test."
"People who had a test processed at the laboratory between 8 September and 4 October will also be contacted and advised to get a test if they have symptoms."
"Our advice to everyone remains that if you have symptoms to self-isolate immediately and book a PCR test," she added.
Public Health Wales said they are "aware" of the issue and added: “We can confirm that we have not identified issues with our own PCR testing results."
What are Lateral Flow and PCR tests?
Lateral flow tests (LFTs) are rapid tests widely used by schools and workplaces to find people with no symptoms who are infected and can spread the virus.
PCR tests, which are sent off to a lab to be analysed, detect the virus several weeks after infection and trigger contact tracing.
After a positive LFT, official guidance is to take a PCR to confirm Covid-19.
Dr Will Welfare, public health incident director at UK Health Security Agency, said: “We have recently seen a rising number of positive lateral flow device results subsequently testing negative on PCR.
"As a result of our investigation, we are working with NHS Test and Trace and the company to determine the laboratory technical issues which have led to inaccurate PCR results being issued to people."
He said there is no evidence of faults with lateral flow tests or PCR test kits.
Andrea Riposati, CEO of Immensa Health Clinic Ltd, said the company is "fully collaborating with UK Health Security Agency on this matter.
"We do not wish this matter or anything else to tarnish the amazing work done by the UK in this pandemic," she added.