More than 40,000 residents in the South West may have been given false-negative PCR test results due to a laboratory mix up, the North Somerset Council has warned.
Results from tests received between September 8 and October 12 have been affected, with NHS Test and Trace contacting those who may still be infectious.
The council's executive for health has told the Mercury that it has been in close contact with the UK Health and Security Agency (UKHSA) and assured residents this is an isolated incident.
Cllr Mike Bell said: "We are seeking assurances from UKHSA about the action they are taking to deliver accurate PCR results to residents.
"Regular testing remains an important part of our response to Covid-19 and local management and, while we have been assured this is an isolated incident, we all need to have confidence in the test and trace system."
The error has been linked to a facility based in Wolverhampton but has mainly impacted tests sent from the South West.
The director of public health for North Somerset Council has advised anyone with Covid symptoms to remain 'extra cautious' for the time being.
Matt Lenny said: "We understand through our contact with the UKHSA that a proportion of the 43,000 samples are from North Somerset, so it is really important that our residents in North Somerset are being extra cautious if they have symptoms of the virus and have tested positive using a lateral flow test.
"Anyone who gets contacted by NHS Test and Trace and is advised to do so should book another PCR test."
Common symptoms of Covid include a new, continuous cough, high temperature and/or a loss or change of your sense of smell or taste.
Mr Lenny added: We have written to local schools and care homes with an update and guidance on this issue."
The council also stressed that there is no evidence of any faults with lateral flow or PCR test kits themselves.
To book a free PCR test, visit www.gov.uk/get-coronavirus-test
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