Sadiq Khan has warned Londoners that the city faces “a difficult winter ahead” as the capital prepares to enter Tier 2 lockdown.
Household mixing will be banned in London from midnight on Friday.
The capital will move from Tier 1 restrictions – the baseline for England known as medium risk – to Tier Two or high risk restrictions on Saturday.
The Mayor said the new rules are “what is needed to save lives in the capital” based on scientific evidence.
“Nobody wants to see more restrictions but this is deemed necessary in order to protect Londoners by myself, London council leaders and ministers,” he said.
Mr Khan met with Health Secretary Matt Hancock, London leaders and scientific advisors this morning, and said the final details of new restrictions were being decided as he spoke.
The Mayor also supports a short national lockdown – known as a ‘circuit breaker’ – which Government scientific advisors have recommended.
“This could save thousands of lives, drive the virus down to manageable levels and give the Government more time to finally get a grip on its failing test and trace system,” Mr Khan said.
“In London, we experienced the worst of Covid-19 back in the spring,” he added.
“Thousands of lives were lost and our economy has been left reeling. We simply cannot afford for the Government to be slow to act again.”
The Mayor warned Londoners that the capital faces “a difficult winter ahead”.
“But – just has we’ve always done throughout our city’s great history – I know we’ll get through this dark time by pulling together,” he added.
It comes as coronavirus cases across the city approach 100 per 100,000 people, with more than 8,000 new infections a week.
Richmond, the worst hit borough, has more than 133 cases per 100,000, with 124 in Hackney and the City of London taken together, and in Redbridge.
There are 114 confirmed infections per 100,000 in Harrow, 109 in Haringey, 106 in Barnet and 101 in Hammersmith and London.
Several other London boroughs are also close to 100 cases per 100,000, including Hounslow, Kingston, Kensington and Chelsea and Wandsworth.
Health minister Helen Whateley held a briefing with all London MPs to discuss the changes.
The Government is expected to announce further details to Parliament.