Sadiq Khan has blamed the “near collapse of test and trace” as London was added to the national coronavirus watchlist.
There will be no new restrictions in the capital at present, but the city will be monitored as an area of greatest concern.
The Mayor said London is at a “very worrying tipping point” as 111 calls and hospital admissions grow.
Some 620 cases of coronavirus were confirmed in the capital, but weekly testing numbers are down 43% since mid-August.
And contact tracers are still struggling to reach those who could have the virus – with just 70% reached in the best performing borough, Lambeth.
East London is faring worst, with just 51% of at risk people reached in Hackney and the City of London, and similarly low figures across neighbouring areas.
“Testing capacity was diverted away from London in the last two weeks to other national hot spots,” Mr Khan warned.
“The lack of testing capacity is totally unacceptable and it is why London has been added to the Government’s coronavirus watchlist as an area of concern.
“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.”
Mr Khan said London’s new high risk status showed new restrictions brought in nationally this week were “absolutely necessary”.
Following Prime Minister Boris Johnson’s announcement on Wednesday:
- Pubs, bars, and restaurants have a 10pm curfew.
- Only table service is allowed.
- Taxi drivers, waiters, bar staff and shop workers have to wear a mask.
- Only 15 people can attend a wedding, down from 30.
- Plans for fans to go back to sports matches are on hold.
- People cannot meet with those outside their social bubble in groups of more than six.
Social distancing rules remain in place: everyone should stay two metres from others outside their household bubble, or use mitigations to reduce the risk, like keeping contact short and wearing a face mask.