A blunder at a lab which saw thousands of positive Covid-19 cases reported as negative could have led to the deaths of at least 20 people, according to new estimates.

The error at the Wolverhampton lab meant that around 39,000 PCR tests were reported as negative when they should have been positive between September 2 and October 12 2021 – mostly in the south-west of England.

As a result, many people would have continued with their daily lives and not self-isolated even though they had Covid-19.

Experts from the UK Health Security Agency (UKHSA) have estimated that the blunder led to about 55,000 additional infections.

They estimated that each person who received a false negative result went on to infect around two other people on average – though some would have continued to take measures to reduce the spread of infection.

Researchers also estimated that there were about 680 additional hospital admissions “that may not otherwise have occurred”.

“Similarly, we estimate that there may have been just over 20 additional deaths in these most affected areas,” they added.

Supplementary data suggests there could have been 684 additional hospital admissions and 23 extra deaths than would have been expected due to the error.

And in a worst-case scenario the blunder could have led to up to 154 extra deaths, but this estimate is much less likely.

NHS Test and Trace suspended testing operations provided by Immensa Health Clinic Ltd at its laboratory in Wolverhampton in October 2021 following reports of inaccurate results.

An investigation into the blunder, conducted by the UKHSA, concluded that the error occurred because staff at the lab set the threshold levels for reporting positive and negative results incorrectly.

Richard Gleave, UKHSA director and lead investigator, said: “Through this investigation we have looked carefully at the arrangements in place for overseeing contracts of private labs providing surge testing during this time.

“We have concluded that staff errors within Immensa’s Wolverhampton laboratory were the immediate cause of the incorrect reporting of Covid-19 PCR test results in September and October 2021.

“It is our view that there was no single action that NHS Test and Trace could have taken differently to prevent this error arising in the private laboratory.

“However, our report sets out clear recommendations to both reduce the risk of incidents like this happening again and ensure that concerns are addressed and investigated rapidly.”

Jenny Harries, UKHSA chief executive, said: “UKHSA is committed to being a transparent, learning organisation and this means investigating where things have gone wrong and working out how things can be improved.

“I fully accept the findings and recommendations made in this report, many of which were implemented as soon as UKHSA discovered the incident.

“These ongoing improvements will enhance our ability to spot problems sooner where they do arise.

“We are particularly keen to further improve how we work with local partners and directors of public health as rapid incidents like this unfold.”

The Government awarded Immensa a £119 million contract in October 2020 to urgently “develop volume for PCR testing for Covid in line with test and trace requirements”, the contract shows.

The contract did not go to tender under rules allowing urgent responses to the pandemic.

A further £50 million was awarded to Immensa by the Government for additional PCR testing.

Immensa was incorporated as a company in the UK in May 2020.