London has been placed on the government’s coronavirus local lockdown watchlist.
It means stricter restrictions could be imposed in the capital if cases continue to rise.
In a statement on Friday announcing the move, London Councils said it was “a stark reminder” to Londoners about the threat from the virus.
The body, which represents the 32 borough councils and the City of London, said there would be no new rules imposed “at this stage”.
Sadiq Khan said the said London was now “at a very worrying tipping point”.
“The near collapse of test and trace and the resurgence of the virus means new measures to slow its spread were absolutely necessary,” he said.
“Ministers simply have to get a grip. 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.”
Millions of people across the country are already living under local lockdown rules, including a ban on visiting each other’s houses.
A total of 6,634 new cases were recorded across the UK on Thursday – the largest daily count since the beginning of the pandemic.
The rise in cases is a “clear” signal, Public Health England’s Professor Yvonne Doyle said, as she insisted people must follow the stricter measures announced this week in order to help control the virus.
Thursday saw the introduction of new rules across England that mean pubs and restaurants must close at 10pm.
According to data from the Office for National Statistics (ONS) in England between 26 April and 08 September, 6.2% of people tested positive for antibodies, suggesting they had the infection in the past.
The percentage of people testing positive for antibodies is higher in London than in the East Midlands, the South East and the South West of England.
It comes as Boris Johnson faces a rebellion from Tory MPs demanding a greater say for parliament over lockdown restrictions.
But there is mounting disquiet on the government benches at Downing Street’s decision to announce the regulations without giving MPs a say.
Tom Tugendhat, the senior Tory who chairs the Commons foreign affairs committee, warned on Friday morning that “controlling the lives of 65 million people by fiat” was “not sustainable”.