The Government's mass testing rollout has been hit with further controversy after leaked emails revealed that senior officials raised "urgent" concerns about the accuracy of rapid tests.
One senior executive cautioned that between just two and 10 per cent of positive lateral flow tests (LFT) could be accurate.
Twice-weekly rapid tests were made available for everyone in England last Friday through an online ordering system and a new "Pharmacy Collect" service.
The Telegraph previously revealed that pharmacies had been forced to ration supplies after they were instructed to order just one carton per day, equating to enough tests for around 50 people.
The leaked emails, seen by The Guardian, reveal that officials are considering downscaling the rollout due to growing concerns around the accuracy of lateral flow tests.
Ben Dyson, Executive Director of Strategy, NHS Improvement, cited a "fairly urgent need for decisions" on when to "stop offering asymptomatic testing", The Guardian reported.
"As of today, someone who gets a positive LFT result in (say) London has at best a 25 per cent chance of it being a true positive, but if it is a self-reported test potentially as low as 10 per cent (on an optimistic assumption about specificity) or as low as two per cent (on a more pessimistic assumption)," Mr Dyson wrote in an email on April 9, the day the mass rollout launched.
He went on to add that the Department of Health and Social Care (DHSC) would need to decide whether requiring people to self-isolate after a positive LFT, without a PCR test to corroborate the result, "ceases to be reasonable" in areas in which infection rates are low.
The DHSC told The Telegraph it would not comment on leaked emails, but said at times when virus prevalence in the community is low the probability of an LFT being a false positive is higher. It is therefore asking people to confirm their result with a PCR in these cases.
A DHSC spokesman said: "With around one in three people not showing symptoms of Covid-19, regular, rapid testing is an essential tool to control the spread of the virus as restrictions ease by picking up cases that would not otherwise have been detected." The spokesman added there were no plans to halt the rollout of rapid tests.
Figures from Public Health England's Covid dashboard show that 26,145,310 LFTs were carried out in England between March 8 and April 4, providing 30,769 people with a positive result.
Of the positive results, 16,930 were sent to a lab for a confirmatory check using the PCR – polymerase chain reaction – technique. These checks found that 18 per cent (3,034) of the results were negative, while 82 per cent (13,896) were positive.
The data analysis does not reveal how many people received a negative LFT result when they were positive.
Scientists have previously said the mass use of rapid turnaround tests may have the "opposite effect" of what is intended to be achieved because of high rates of false results. They said false results could mean more workplaces and schools are forced to close, with people required to self-isolate when a positive result is returned.
Dr Alexander Edwards, associate professor in biomedical technology at the University of Reading, told The Telegraph that although LFTs are "quick" and easily mass produced, "they're nowhere near as sensitive as a PCR test".
"Technically they don't work as well as we would like, and what that means is that there's always a possibility that you'll be infected and you'll take a lateral flow test and it will be negative," he said.