COVID-19 testing at the Immensa Health Clinic lab in central England has been suspended.
About 43,000 people may have received a false negative test result because of an error at the lab.
The UK Health Security Agency said that it was an "isolated incident."
About 43,000 people in England may have got a false-negative COVID-19 test result because of an error at a private laboratory, a government agency said.
The UK Health Security Agency (UKHSA) said on Monday that testing at the lab in Wolverhampton, central England, had been immediately suspended, pending an investigation.
The potential error was found while the UKHSA was investigating a rising number of people who were testing positive on quick lateral flow tests (LFTs) but negative on lab tests, the UKHSA said.
The lab, run by Immensa Health Clinic Ltd, had processed around 400,000 samples, the UKHSA said. An estimated 43,000 people may have been given an incorrect negative test result between September 12 and October 8, mostly in the south-west of England, it said.
It was investigating the precise cause, it said, adding that it was an "isolated incident."
"The number of tests carried out at the Immensa laboratory are small in the context of the wider network and testing availability is unaffected around the country," it said.
The National Health Service (NHS) was contacting the people who may have COVID-19 to tell them to take another test, the UKHSA said.
Dr. Will Welfare, public health incident director at UKHSA, said in a statement that the UKHSA was working with Immensa and the UK central testing body, NHS Test and Trace, to determine the "laboratory technical issues" that led to the possible error.
There was "no evidence" of any fault with the quicker lateral flow tests or lab tests more generally, Welfare said. "The public should remain confident in using them and in other laboratory services currently provided," he added.
Andrea Riposati, chief executive director at Immensa Health Clinic Ltd, said in a statement that the company was "fully collaborating" with the UKHSA. "We do not wish this matter or anything else to tarnish the amazing work done by the UK in this pandemic," she said.
Alexander Edwards, associate professor in Biomedical Technology at the University of Reading, said in a statement that we shouldn't assume all problems or failures mean that testing is bad.
"Let's not throw the baby out with the bathwater. The majority of test results are correct, and it's worth remembering that our testing system has been built up from almost nothing at the start of the pandemic," Edwards said.