Britain reported 12,872 new COVID-19 cases on Saturday, a record daily figure for the country, with the government blaming the jump on a reporting delay and saying the numbers in coming days would include additional cases.
The government's website said a technical issue had delayed publishing a number of new COVID-19 cases.
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"This means the total reported over the coming days will include some additional cases from the period between 24 September and 1 October, increasing the number of cases reported," it said.
At 12,872, the number of daily cases reported almost doubled from the 6,968 reported on Friday, smashing the previous biggest daily toll of 7,143, which was recorded on Tuesday.
Britain is now processing more than 200,000 COVID-19 tests a day compared with fewer than 100,000 at the start of the pandemic.
But the technical issue could be seized on by critics who say the government has lost control of the virus amid a shortage of testing capacity.
The number of deaths recorded within 28 days of a positive COVID-19 test fell to 49 on Saturday from 66 the previous day.