NHS staff are to be given new coronavirus tests after a minister admitted the initial results were inaccurate.
Care minister Helen Whately announced on Wednesday morning that those who had received inaccurate tests were being notified.
She told Sky News: "My understanding from the clinical advisers is that some of the early tests were evaluated and the evaluation is that they weren't effective enough.
"This is a normal process when you are using a test for an illness when this is a new illness.
"Those that were tested with the test that wasn't up to scratch have been written to and offered another test."
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Whately refused to answer when asked whether workers in hospitals had been cleared to work on the frontline despite being infected.
She said: "We have to make sure we look at the reliability of tests.
"This is really really important, not just to test but to make sure we test people effectively.”
Shadow health secretary Jon Ashworth described the admission as “really worrying”.
He said: “I think ministers need to be absolutely clear about how many NHS staff they think have had false tests and what the plan is now to contact those staff so they can be tested again.
“It is worrying because we were told that the tests developed by Public Health England were world-leading.”
Whately’s admission comes as it was revealed Public Health England (PHE) sent a memo earlier this month that warned about the potential inaccuracies of the tests.
It said there had been “discordant results” in the existing tests and that they were to be replaced with commercial ones.
Some 100,000 NHS and social care staff and their relatives have so far been tested for COVID-19 so they are able to work on the frontline without fear of infecting vulnerable patients.
Ashworth said: “It shouldn't have really been revealed because someone had leaked a memo. Ministers should have been upfront about this.”
PHE called reports that the in-house check had failed to detect a positive diagnosis 25% of the time "inaccurate".
The agency said the COVID-19 test had produced different results to an alternative test in "less than 2% of samples", and that advice was issued to laboratory staff to ensure the test continued to be reliable.
Professor Sharon Peacock, director of the national infection service at PHE, said: "The test is regularly and thoroughly reviewed to make sure it remains reliable and effective.
"It is standard practice to move to commercial test kits once available, and this work is already underway."
The government is still only around a fifth of a way towards its COVID-19 target of 100,000 tests per day across the UK, even as an expert said Northern Ireland is ready to begin a contact-tracing pilot to curb spread of the virus.
Less than half of the available coronavirus testing capacity has been used, according to the latest figures, with fewer than 20,000 tests conducted in the 24 hours to 9am on Monday.
That is despite there being capacity for 39,250 tests to have been carried out over the same period and a deadline of 100,000 tests per day set for the end of this month,
Downing Street insisted Boris Johnson – who is continuing his recovery from COVID-19 – had full confidence in health secretary Matt Hancock and the testing target.
Yahoo News has contacted PHE for comment.