A "technical glitch" in England's test and trace system has resulted in major delays in handing over the details of almost 16,000 positive cases to contact tracers.
Public Health England said 15,841 positive tests carried out between 25 September and 2 October had been added to the UK's daily case totals over the weekend after a computer glitch meant they were not recorded on time.
The blunder saw the UK's positive case figures soar after the backlog was finally added to the official tally, with 12,872 cases recorded on Saturday and a further 22,961 on Sunday.
PHE insisted all those who had been tested during the period had recieved their results as normal, with those testing positive for the virus being asked to self-isolate.
But they conceded the delay meant the close contacts of those confirmed to have the virus were not contacted by the NHS test and trace system.
Asked on Monday how many people may have been missed by contact tracers, prime minister Boris Johnson said: "I can't give you those figures, but what I can say is that all those people are obviously being contacted.
"The key thing, as I say, is that everybody, whether in this group, or generally should self-isolate. That is the way to make it work."
Michael Brodie, the interim head of Public Health England, said the "technical issue" had been spotted on Friday evening, meaning contact tracers were not handed the details until 1am on Saturday morning.
It is believed the glitch was caused by data files recording positive test results exceeding the maximum file size allowed by the system.
"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. The majority of these cases occurred in the most recent days," Brodie said.
"Every one of these cases received their Covid-19 test result as normal and all those who tested positive were advised to self-isolate.
"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
"We fully understand the concern this may cause and further robust measures have been put in place as a result."
Joint medical director Dr Susan Hopkins added: "The delays are in reporting to the dashboard and to the public and there's been a delay in contact tracing initiation.
"Public Health England apologise that this occured and have put in place steps to prevent this happening."
Johnson admitted the delay had been caused by a "failure in the counting system".
On Sunday, he said: "All the people who had a positive test have now been notified and I think the data that we have is realistic, and again it's very useful in helping us to identify... where the incidence is and what we need to do to tackle it."
But speaking on Sunday evening Labour's shadow health secretary Jonathan Ashworth demanded an explanation from health secretary Matt Hancock, saying the "shambolic" delay would leave people "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," he said.