About 16,000 cases of coronavirus have not been entered into the national computer system used for official diagrams due to technical defects, Public Health England said.
Some unreported cases then joined the 12,872 new cases on Saturday and 22,961 on Sunday.
PHE said those who tested positive were generally reported.
But that means there has been a delay in finding their contacts who have been exposed to the virus.
The BBC’s health editor, Hugh Pim, said the daily figures for the weekend were “actually closer to 11,000” than the nearly 1,000,000 reports.
According to the PHE, cases between September 25 and October 2 were dropped from the daily reports.
However, he insisted that the IT problem has been resolved and now all cases have been handed over to the investigation and trace system.
The laborer described the error as “symbolic”.
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Meanwhile, Kate Benham, head of the government’s vaccination task force, told the Financial Times that less than half of the UK population could be vaccinated against coronavirus.
“People under the age of 18 are not going to be vaccinated,” he said. “It is an adult-only vaccine for people over the age of 0, with a focus on health workers, care home workers and vulnerable people.”
Prime Minister Boris Johnson has warned that as much as the UK is dealing with coronavirus, it could be “from Christmas to Christmas”.
Speaking to the BBC’s Andrew Murr on Sunday, the prime minister said there was “hope” for Kovid to be killed and called on people to “act fearlessly but with common sense”.
At a time when the testing system has come under intensive scrutiny after reports of delays and a system is struggling to keep up with demand, the latest revelation could not come at a more troubling moment for the government at Westminster.
Because about 16,000 additional positive test results were not entered into the test and trace system, their recent contacts were not immediately followed.
Experts advise that contacts should ideally be found within 48 hours.
Officials say a perceived technical problem related to IT has been resolved, adding all the new cases registered over the weekend.
But all of this will hardly improve public confidence in England’s testing system.
And it pollutes the water for policymakers and officials trying to track down the spread of the virus known as the “critical moment” to the Prime Minister.
On Sunday, the government’s Coronavirus Dashboard said that as of 09:00 BST, there have been a further 22,961 lab-confirmed cases of coronavirus in the UK, bringing the total number of cases in the UK to 502,978.
As of Sunday, another 33 people had died within 28 days of testing positive for Covid-19.
The note on the dashboard states: “The cases published from October to October include 15,841 additional cases with sample dates between September and October – so it is artificially high for England and the UK.”
Michael Brody, interim chief executive of Public Health England, said the process of transferring Covid-19 positive lab results to reporting dashboards identified a “technical issue” on Friday night, October 2.
“After a quick investigation, we have identified that 15,841 cases between September 25 and October 2 have not been reported in the daily Kovid-19 case. Most of these cases have occurred in recent days.”
“Each of these cases received a result of their Covid-19 test as normal and those who tested positive were advised to self-isolate.”
He said they “worked with the NHS test and trace to resolve the issue quickly and transferred all remaining cases to the NHS test and trace contact tracing system immediately.”
“We fully understand the concerns raised by this and as a result, tougher measures have been taken,” he said.
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Susan Hopkins, a joint medical consultant at Test & Trace and Public Health England, added that a “complete health risk assessment was conducted to prioritize pending cases for effective detection of contact.”
The PHE said NHS tests and traces have ensured that there are adequate contact tracers in operation, and are working with local teams to ensure that they also have sufficient resources to be able to contact all cases urgently.
The number of call attempts is being increased from 96 to 10 to 15.
Jonathan Ashworth, Labor’s shadow health secretary, said: “This is symbolic and people across the country will be amazed.
“(Health Secretary) Matt Hancock should come to the House of Commons on Monday to discuss what has happened on Earth, what has affected our ability to contain this virus, and what he plans to do to improve testing and trace.”
A health department spokesman said the issue did not affect people who received test results, and those who tested positive were generally informed.
News of the daily countdown first surfaced late on Saturday, when the UK announced more than 10,000 new coronavirus cases for the first time since mass testing began.
The government said the technical problem meant that some cases were not reported during the week at the time, so these were included in Saturday’s data.
The daily total rose sharply from 4,044 on Monday to 7,143 on Tuesday. However, the daily total remained stable over the next four days – at a time when a steady increase could be expected – fluctuating between 6,914 and 7,108.
Then came the big leap in numbers – a much higher day-to-day increase than at any other time in the entire epidemic – which was announced on Saturday, five hours later than usual, and with the government’s explanation.
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