A technical glitch that meant nearly 16,000 cases of coronavirus went unreported has delayed efforts to trace contacts of people who tested positive.
Public Health England said 15,841 cases between 25 September and 2 October were left out of the UK daily case figures.
PHE said all those who tested positive had been informed. But it means others in close contact with them were not.
The issue has been resolved, PHE said, with outstanding cases passed on to tracers by 01:00 BST on Saturday.
The technical issue also means that the daily case totals reported on the government’s coronavirus dashboard over the past week have been lower than the true number.
BBC health editor Hugh Pym said daily figures for the end of the week were “actually nearer 11,000”, rather than the about 7,000 reported.
Labour has described the glitch as “shambolic”.
Meanwhile, the head of the government’s vaccine taskforce, Kate Bingham, has told the Financial Times that less than half of the UK population could be vaccinated against coronavirus.
“There is going to be no vaccination of people under 18,” she said. “It’s an adult-only vaccine for people over 50, focusing on health workers, care home workers and the vulnerable.”
Prime Minister Boris Johnson has warned it could be “bumpy through to Christmas” and beyond as the UK deals with coronavirus.
Speaking to the BBC’s Andrew Marr on Sunday, the PM said there was “hope” of beating Covid, and called on the public to “act fearlessly but with common sense”.
In an interview with the Sun, Rishi Sunak has defended his Eat Out to Help Out scheme, saying he had “no regrets”, after suggestions it may have helped fuel the second wave of coronavirus cases.