Britain has recorded its highest daily total of coronavirus cases for nearly three months, heightening fears that a second wave is imminent.
The toll of 1,522 new infections is the biggest since June 12, three weeks before ministers lifted restrictions on pubs and restaurants.
The total threatens to dash hopes that had been growing in recent days that the steady increase in infections since the lifting of lockdown was finally under control.
However, sources at the Department of Health and Social Care (DHSC) put the sudden rise in cases down to specific isolated outbreaks in factories and other workplaces, rather than a more widespread trend.
At least 40 coronavirus outbreaks have been reported in food processing plants, including a Greggs distribution site in Leeds and a chicken factory in Norfolk where 75 staff have tested positive.
The new DHSC figures threaten to reignite doubts over the reliability of government data, coming just hours after NHS Test and Trace announced the first weekly decrease in positive cases for six weeks.
The programme, which aggregates testing data from hospitals, mobile testing sites in the community and home kits, recorded 6,115 positive results in the week up to August 19.
The total was down 7.6 per cent on the previous seven days, despite a two per cent increase in the number of tests carried out.
Despite the hike in new infections as reported by DHSC, officials announced only 12 new deaths on Thursday.
This could mean that younger people are predominantly driving the rise in new infections.
Ministers and officials can access real-time data on the location of every new case, and on Friday Matt Hancock, the Health Secretary, is expected to announce highly targeted increases in restrictions in a small number of areas including Swindon, where there have been several recent outbreaks.
Local lockdowns in some other parts of the country are expected to be eased slightly, with the majority of lockdown measures remaining the same.
One Government source said: “It’s too early to say whether this rise is part of trend.
"However, we know increases are happening in the rest of Europe which is why local lockdowns are so important.
“We’re seeing the numbers fall in the lockdown areas but there have been some isolated outbreaks elsewhere.”
Meanwhile, council leaders in locked-down areas of the North have warned that their residents’ patience is “wearing thin” with social distancing rules.
Under current measures, visits between households are largely banned, but people are still permitted to go to the pub.
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In a statement, councillors Susan Hinchcliffe, Tim Swift and Shabir Pandor, the leaders of Bradford, Calderdale and Kirklees respectively, said: "Our infection rates have come down significantly this week, although they are still high.
"Over the last few months and weeks we, as leaders, have taken difficult decisions and directed significant resource in our councils to make this happen.
"However, throughout this time Government messaging has been confused and bungled.
"The latest shift in Government messaging that happened last week was that they inferred they wanted to see more localised restrictions, varying them ward by ward rather than by local authority areas.”
The councillors went on: “Adding and subtracting restrictions ward by ward makes the already confused local regulations almost impossible to understand for residents so it begs the question whether restrictions across partial geography can be of any use at all?
“On top of this, people's patience is wearing thin with the confusion. They need to know that the restrictions are fair otherwise they won't keep to them.”
Kevin McConway, emeritus professor of applied statistics at The Open University, said that the optimistic picture painted by Thursday’s Test and Trace figures correlated with trends reported by the ongoing Office for National Statistics (ONS) survey.
While based on a smaller set of data, it is more resilient to one-off factors that can misrepresent results due to being carefully weighted across the country.
“Because this is a survey and there is inevitable statistical uncertainty in the estimates, we can’t be absolutely sure that the slow rise has levelled off,” he said.
“But it’s certainly reassuring to see the same sort of pattern in the Test and Trace new positive tests as in the ONS infection survey results – new infections lowest at the end of June, a slow rise after that, but then levelling off.”
New ONS data for Covid-19 is due to come out on Friday.