London has been added to the government's Covid-19 watch-list following a rise in cases in the city, officials have said.
All boroughs have been classed as areas of concern, but no additional restrictions have been announced.
Mayor of London Sadiq Khan said the city was at "a worrying tipping point" with hospital admissions increasing.
Councils in the city have urged residents to abide by current restrictions.
The watch-list, published each week, categorises local councils seeing a higher infection rate as "areas of concern", "areas of enhanced support" or "areas of intervention".
Tighter restrictions are usually introduced for areas in the third category.
Earlier on Friday, London Councils - the group representing all 33 local authorities in the city - announced the city would be joining the list, saying it was a "stark reminder" to residents they must follow new rules announced this week.
The organisation called for a sustained boost to Covid-19 testing in the capital so infections can be monitored.
Mr Khan said testing capacity had been "diverted away" from the capital to other national hotspots, causing the number of tests in the city to drop by 43%.
"The lack of testing capacity is totally unacceptable," he said.
"It's vital that testing capacity is increased immediately in London and focused in the areas it is needed most. Any delay will mean letting the city down and will cost lives."
The most recent daily figures showed 584 coronavirus cases were reported across London. In the week to 21 September, Redbridge had the highest infection rate per 100,000 residents compared with other boroughs, at 55.4.