The prime minister has been unable to confirm how many coronavirus contacts were missed as a result of a glitch in the testing system which stopped almost 16,000 cases from being reported.
The delay in reporting new cases means thousands of people who were potentially infected with coronavirus were not told to self-isolate for at least a couple of days and the infection may have been seeding more widely as a result.
But Boris Johnson told reporters in central London he could not reveal exactly how many contacts had been missed.
"I can't give you those figures," he said, "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."
Explaining the missing cases, he said: "What happened here was that some of the data got truncated and it was lost.
"But what they have done now is not only contacted all the people who were identified as having the disease - that was done in the first place - but they are now working through all the contacts as well."
ITV News Political Editor Robert Peston said this situation was "at odds with Test and Trace’s initial diagnosis of the computer glitch 24 hours earlier".
Labour has branded the situation "shambolic".
Public Health England (PHE) said a technical issue resulted in 15,841 cases between September 25 and October 2 being left out of the reported daily coronavirus cases.
Senior officials said the outstanding cases were transferred to NHS Test and Trace “immediately” after the issue was resolved and thanked contact tracers for their “additional efforts” over the weekend to clear the backlog.
All cases were passed on to tracers by 1am on Saturday, meaning potential delays of more than a week in contacting thousands of people who were exposed to the virus and telling them to self-isolate.
PHE said every single person who was tested initially had received their test result as normal, with all those testing positive told to self-isolate.
The technical issue – caused by some data files reporting positive test results exceeding the maximum file size – also means that daily totals reported on the government’s coronavirus dashboard over the last week have been lower than the true number.
For example, 4,786 cases which were due to be reported on October 2 were not included in the daily total on the dashboard that day, when the figure was given as 6,968.
The government’s dashboard said that, as of 9am on Sunday, there had been a further 22,961 lab-confirmed cases of coronavirus in the UK, taking the total number of cases in the UK to 502,978.
A note on the dashboard said: “The cases by publish date for 3 and 4 October include 15,841 additional cases with specimen dates between 25 September and 2 October – they are therefore artificially high for England and the UK.”
Michael Brodie, interim chief executive at PHE, said the “technical issue” was identified overnight on Friday, October 2, during the transfer of positive coronavirus test results in to the dashboard.
“NHS Test and Trace and PHE have worked to quickly resolve the issue and transferred all outstanding cases immediately into the NHS Test and Trace contact tracing system and I would like to thank contact tracing and health protection colleagues for their additional efforts over the weekend,” he said.
Test and Trace and PHE joint medical adviser Susan Hopkins said: “All outstanding cases were immediately transferred to the contact tracing system by 1am on October 3 and a thorough public health risk assessment was undertaken to ensure outstanding cases were prioritised for contact tracing effectively.”
PHE said NHS Test and Trace have made sure that there are more than enough contact tracers working, and are working with local health protection teams to ensure they also have sufficient resources to be urgently able to contact all cases.
The number of call attempts is being increased from 10 to 15 over 96 hours.
Shadow health secretary Jonathan Ashworth branded the situation "shambolic", adding: said: “People across the country will be understandably alarmed.
“Matt Hancock should come to the House of Commons on Monday to explain what on earth has happened, what impact it has had on our ability to contain this virus and what he plans to do to fix test and trace.”
Labour MP Bridget Phillipson reiterated her party's rhetoric on the matter to ITV News.
Bridget Phillipson says delay in contacting people is "shambolic"
On Monday morning, Work and Pensions Secretary Therese Coffey said it was key that PHE spotted the error and are working to resolve it.
"I think PHE have admitted there's been a glitch, I don't know the exact source of the glitch, but the important thing is that PHE have also identified what had gone wrong and are now fixing that error," Ms Coffey said.
"I think the overall important element is that the people who had the actual tests got the results and from that could either decide or be instructed in effect to self-isolate or not."
Therese Coffey does not know how the data was lost
On Saturday, Professor Graham Medley, an attendee of the Scientific Advisory Group for Emergencies, tweeted: “Reporting delays play havoc with data streams and make them very difficult to analyse in real time.
“If the delays change or vary by group then they can distort a lot. Wonder what these will do to the R estimates next week”.
Meanwhile, the head of the country’s vaccine taskforce, Kate Bingham, told the Financial Times that less than half of the UK population could be vaccinated against Covid-19.
The head of the immunisation programme told the newspaper: “People keep talking about ‘time to vaccinate the whole population’ but that is misguided.
“There is going to be no vaccination of people under 18.
“It’s an adult-only vaccine for people over 50, focusing on health workers and care home workers and the vulnerable.”
Elsewhere, Tim Spector, professor of genetic epidemiology at King’s College London, said the effects of long Covid could turn out to be a bigger public health problem than excess deaths.
A report from the Tony Blair Institute for Global Change is recommending that the government highlight the issue in awareness campaigns.
In relation to future restrictions, the Guardian reported that a three-tier lockdown is planned for England which may include the closure of pubs and a ban on all social contact outside household groups.
A government spokesperson said: “We are seeing coronavirus cases rise at a rapid rate across the country and, given how serious this virus is, it is vital everyone plays their part by following the rule of six, washing their hands, practising social distancing and wearing a mask in enclosed spaces.
“As we have shown, we are prepared to take action decisively when it is necessary, and it is of course right to look how we make sure everyone understands and complies with the restrictions that will keep us all safe.”