People who got a negative result from a Covid testing site in Berkshire earlier this month are being told to book another test, amid fears they were mistakenly given the all-clear.
Some PCR tests carried out at Newbury Showground resulted in false negatives, West Berkshire Council said.
The BBC has been told the problems relate to one specific lab, rather than the site, and have now been fixed.
But some other people in south-west England are thought to be affected too.
Health officials are set to give out more details later on Friday.
In a statement issued on Thursday evening, the council said it had recently received reports from local residents who were concerned about the accuracy of tests taken at Newbury Showground.
The coronavirus testing site - an events venue in a village in the outskirts of Newbury - is operated by the government.
Most PCR tests are taken at sites such as this and the samples are sent off to a lab for analysis, with the results then communicated to the tested person, usually by text or email, within 72 hours.
West Berkshire Council said: "Over the past month, some PCR tests completed at the Newbury Showground testing site have had results sent out that may have incorrectly shown as negative for Covid-19.
"If you took a PCR test between 3 and 12 October which was negative, we strongly recommend a retest for you and for any close contacts."
Other sites possibly affected
The council also said it had been told by the government that "a number of sites nationally may have been affected by this issue, including the one at Newbury Showground".
It comes after widespread reports in south-west England of people testing positive with lateral flow tests, but then later testing negative when they got a PCR test.
GPs said they were seeing a stream of patients who had received positive lateral flow tests, who then went on to receive negative PCR results.
The UK's Health Security Agency - which used to be Public Health England - said it had been made aware of "some areas across the country" encountering this issue - and that it was investigating.
PCR tests are generally typically considered to be the most accurate type of test - although new research this week showed lateral flow tests are more reliable than first thought.
Government guidance says people must get a follow-up PCR test after a positive LFT to confirm they have Covid - and they can end their self-isolation when they get a negative result in a PCR test.
Public health officials are set to give more details later of how many people may have got false results and how they will be contacted.