Boris Johnson's mass Covid-19 testing plan hit another hurdle after leaked emails showed officials raised urgent concerns about the accuracy of rapid test kits.
The prime minister last week urged everyone in England to take two rapid tests a week as the country comes out of lockdown.
The scheme is one of the biggest expansions of the multibillion-pound testing regime to date.
However, officials are now understood to be considering winding back the testing of people without symptoms with lateral flow devices because of growing concerns over the number of false positive results.
An email leaked to The Guardian newspaper estimates that as few as 2 per cent to 10 per cent of positive results are accurate.
Ben Dyson, an executive director at NHS Improvement, cited a “fairly urgent need for decisions” on when to “stop offering asymptomatic testing”.
“As of today, someone who gets a positive LFD 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 2 per cent (on a more pessimistic assumption),” he wrote in an email on April 9, the day the mass distribution of the tests began.
He said that officials would need to decide at what point asking someone to self-isolate for 10 days after a positive result from the test kits “ceases to be reasonable”.
Prof John Simpson, from Public Health England, suggested there was not enough evidence to suggest the scheme would reduce the spread of the virus.
“We are a little concerned,” he wrote.
Britain’s Department for Health said there were no plans to halt the distribution of rapid tests.
“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,” it said.
Mr Johnson said last week rapid tests were a key pillar of the government's post-lockdown strategy, as were vaccinations.
“As we continue to make good progress on our vaccine programme and with our road map to cautiously easing restrictions under way, regular rapid testing is even more important to make sure those efforts are not wasted,” he said.
“That’s why we’re now rolling out free rapid tests to everyone across England – helping us to stop outbreaks in their tracks, so we can get back to seeing the people we love and doing the things we enjoy.”