Leeds has been added to the official watchlist of “area of concern” after a rise in coronavirus cases.
The problem has been partly blamed on young people attending gatherings and house parties.
The city council urged residents to “do their bit” to stop COVID-19 spreading ahead of the weekend.
It said the virus case rate is now 32.4 per 100,000 people and infections are spread across different parts of Leeds “meaning they may be linked to social interaction and leisure activities”.
The council added coronavirus is being “increasingly detected in younger people aged 18-34, with some concern over activities like house parties and gatherings”.
Andrew Carter, a local councillor who heads the Conservative group, told Sky News the change was “quite right” as there has been a “whiff of complacency”.
He complained parts of the city have been “left without much testing locally at all” – with one drive-through centre running “way below capacity”.
It comes as the virus growth rate rose slightly from being between -2% and 1% to between -1% and 2% per day.
While the UK-wide R number – the average number of people someone with COVID-19 passes it on to – stayed the same at between 0.9 and 1.1.
The “areas of concern” list is published every Friday, revealing how coronavirus is spreading broken down by region.
Last week, Bury was one of the places where cases notably rose – up to 31.6 per 100,000 people.
But lots of other areas already facing greater restrictions saw cases decreasing, including Pendle, Oldham, Blackburn with Darwen, Manchester, Rochdale and Salford – although they remained at a high level relative to other parts of the country.
That led to the government initially saying it would ease tighter measures in Bolton and Trafford.
But after calls not to by the local councils, ministers U-turned and kept rules the same, meaning residents cannot socialise with other households in their own homes, gardens, pubs or restaurants for the foreseeable future.
Bolton and Corby have been identified by scientists at Imperial College London as the potential next COVID-19 hotspots.
Analysis: Is the glass half-empty or half-full?
By Rowland Manthorpe, technology correspondent
At the height of the pandemic, the government made a concerted effort to educate the public about the importance of the R number. When it stays below 1, we were told, everything is going well.
So is today’s news that the R for England has remained between 0.9 and 1.1 for the third successive week a cause for celebration?
In truth, it’s hard to say.
With a range like that, it could be rising, falling or staying stable, so it very much depends on your perspective.
For glass-half-full types it’s reassuring. For those who see a glass as half empty, it’s disappointing.
The reality is that, at low prevalence, the R number is weak guide to the state of the outbreak.
Go through the minutes of SAGE and you’ll see the government’s scientific advisers making this point repeatedly.
One senior scientist admitted that by encouraging people to focus on it, “we’ve created a monster.”
It may be a little boring, but we should react to today’s news in England the same way we react to yesterday’s rise in Scotland: by not reading into it too much.