An investigation is under way after nearly 16,000 coronavirus cases went unreported in England, delaying contact tracing efforts.
Downing Street said there was a “technical issue” when positive test results were transferred from labs to central dashboards.
Public Health England said 15,841 cases between 25 September and 2 October were left out of the UK daily case figures.
Those who tested positive were informed – but their close contacts were not.
Experts advise that ideally contacts should be tracked down within 48 hours.
The issue has been fixed, PHE said, with outstanding cases passed on to tracers by 01:00 BST on Saturday.
The prime minister’s spokesman said the glitch was “quickly resolved” and NHS Test and Trace and PHE were “urgently contacting” all cases, with extra contact tracers being used.
However, he said an investigation has been launched into why the problems were not picked up sooner.
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The technical issue meant 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 roughly 7,000 reported.
Labour described the glitch as “shambolic”.
It was caused by some data files exceeding the maximum size, and issues in transfer of data between NHS Test and Trace and PHE.
Health Secretary Matt Hancock is due to update Parliament on Covid-19 on Monday afternoon.
Source: BBC World