Technical problems that led to almost 16,000 COVID-19 positive test results being left off official figures delayed efforts to track and trace contacts.
Public Health England (PHE) said on Sunday that the data problem had now been resolved.
Michael Brodie, PHE's interim chief executive, said: "A technical issue was identified overnight on Friday 2 October in the data load process that transfers COVID-19 positive lab results into reporting dashboards.
"After rapid investigation, we have identified that 15,841 cases between 25 September and 2 October were not included in the reported daily COVID-19 cases."
Mr Brodie said every individual during this period received their COVID-19 test result as normal, "and all those who tested positive were advised to self-isolate".
Test and Trace Delays
It is understood that the problem stemmed from some Excel data files having too many rows for the old format.
The glitch meant that testing figures were artificially high for 3 and 4 October and under reported in the preceding week.
Crucially, it led to a delay in tracing those who had been in close contact with people who tested positive for COVID-19.
The missing cases were added to Saturday's and Sunday's figures, bringing them up to 12,872 and 22,961 new cases.
Asked on Sky News this morning how many people had not been traced as a result of the delay in reporting infections, Prime Minister Boris Johnson said: "I can't give you those figures.
"What I can say is all those people are obviously being contacted and the key thing is that everybody – whether in this group or generally – should self-isolate."
According to PHE, 75% of cases that should have been reported occurred between 30 September and 2 October.
"All outstanding cases were immediately transferred to the contact tracing system by 1am on 3 October and a thorough public health risk assessment was undertaken to ensure outstanding cases were prioritised for contact tracing effectively," said Dr Susan Hopkins from Imperial College London, test and trace and PHE joint medical advisor.
England's Health Secretary Matt Hancock told the Commons this afternoon it was "an ongoing incident". Extra staff had been brought into help catch up contact tracing, and that the Joint Biosecurity Centre confirmed "that this has not impacted the basis on which decisions about local action were taken last week."
He added: "This incident should never have happened. But the team have acted swiftly to minimise its impact. and now it is critical that we work together to put this right and to make sure that it never happens again."
'An Utter Mess'
Labour's Shadow Health Secretary, Jonathan Ashworth, commented: "This is shambolic and people across the country will be understandably alarmed."
Karol Sikora, professor of medicine at the University of Buckingham Medical School, who comments on cancer issues for Medscape UK, tweeted that he was angry at the way data was being handled. "Life and death decisions are based on these numbers, and it is an utter mess," he tweeted.
Rowland Kao, professor of veterinary epidemiology and data science at the University of Edinburgh, explained to the Science Media Centre: "All those individuals with positive results that were not entered into the system have contacts who remained an infection risk to others over this period and so we can expect that they will have already contributed extra infections which we shall see over the coming week or so.
"While it appears they are now being contacted as a matter of priority, this additional strain on a system already stretched to its limit implies that further delays are likely to occur for other cases where contact tracing is needed.
"These knock-on effects may have a substantial influence on the generation of new cases, over a period even longer than that."
Prof Kao predicted that further glitches were likely to occur with a complex system that was put into place at short notice.
Prof Michael Hopkins from the Science Policy Research Unit at the University of Sussex Business School said the centralised UK system was vulnerable to system-wide problems.
"By contrast the German model for test and trace is much more locally based and would not suffer from this kind of problem," he said.
Nigel Marriott, a fellow of the Royal Statistical Society, said: "The big problem with this latest data issue from PHE is that we were misled as to the underlying trend during that period.
"Up to Friday it looked like the recent surge in cases had paused and there was hope of a turnaround in some places. But with the revisions, it is clear that there is still a strong upward trend and more measures may be needed to reverse the trend."
Daily Data
In today's daily data another 12,594 UK positive tests were reported and 19 deaths.
There are 2428 COVID-19 patients in hospital and 368 ventilator beds are in use.
Three Tier Lockdown System: Report
It was reported today that the Government is planning to launch a new three tier 'traffic light' system on Thursday to tailor lockdown measures in areas of England according to the severity of coronavirus infections.
The Guardian said it had seen leaked documents suggesting that the draft plans were designed to simplify the current patchwork of local restrictions.
The report said the most serious restrictions – alert level 3 – included closure of hospitality and leisure businesses, banning people from meeting others outside their own household, and restrictions on overnight stays away from home.
Under alert level 2, people would not be able to meet others outside of their household in private dwellings or gardens, apart from their support bubble, or in pubs, restaurants, or other settings.
The paper said the Covid-19 Proposed Social Distancing Framework had yet to be signed off by Number 10.
On Sunday, Mr Johnson warned that it could be "bumpy through to Christmas" and beyond because of the pandemic.
