Staff mistakes in a private laboratory may have caused 23 extra deaths from Covid-19.
The UK Health Security Agency (UKHSA) made the claim in a report into errors at the Immensa lab in Wolverhampton.
It said as many as 39,000 positive results were wrongly reported as negative in September and October 2021.
The mistakes led to "increased numbers of [hospital] admissions and deaths", the report, published on Tuesday, concluded.
Thousands of people, many in the South West, were wrongly told to stop testing after their results were processed by Immensa.
The Wolverhampton laboratory was used for additional testing capacity for NHS Test and Trace from early September 2021, but testing was suspended on 12 October following reports of inaccurate results.
Experts said high case rates in some areas were down to people unwittingly infecting others when they should have been isolating.
UKHSA experts said the mistakes could have led to as many as 55,000 additional infections in areas where the false negatives were reported.
"Each incorrect negative test likely led to just over two additional infections," the report said.
"In those same geographical areas, our results also suggest an increased number of admissions and deaths."
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Source: BBC News, 29 November 2022