London's mayor has urged the government to ask all secondary schools and colleges in the capital to shut early ahead of Christmas.
In a letter to ministers, Sadiq Khan said he also wanted schools to reopen later in January amid "significant" Covid outbreaks in 10 to 19-year-olds.
It comes as the BBC was told London was likely to move into tier three.
Greenwich and Islington councils are the first in England to urge schools to switch this week to online learning.
Council officials in Greenwich have advised schools to shut from the end of Monday, although some academies will remain open, while Islington schools have been asked to move online from the end of Tuesday.
A spokesman for the Department for Education said it was "vital" children remained in school until the end of term.
The prime minister's official spokesman said not being in school had "a detrimental impact on learning and other areas of pupils' development".
Regional school commissioner teams were working closely with local authorities to" keep schools open and keep pupils and staff safe," he added.
Mr Khan described the surge in Covid-19 cases in London "deeply concerning" and that in the last week, there had been a 75% increase in those aged 10-19 testing positive for the virus.
He said "if the government isn't careful these children will pass on the virus to really vulnerable people because the rules are relaxed" over Christmas.
"My message to the government is if you can't keep the schools Covid safe in the last few days before Christmas, it's better to err on the side of caution and revert to online teaching for these few days."
'Very dark tunnel'
In the letter, also sent to the prime minister, he said he wanted regular asymptomatic testing to be extended to everyone who could not work form home as well as students and staff at London's secondary schools, sixth-form college and further education colleges.
He has also called for face coverings to be made mandatory in busy outdoor public spaces, "given the numbers on our high streets in the run-up to Christmas".
"The rollout of the vaccine has provided some light at the end of what has been a very dark tunnel, but this is no time to be complacent and we cannot let so many months of compromise and sacrifice go to waste," he said.
"Time is running out to get the virus under control in our city which is why I urge the government to heed my call and provide us with the extra support we desperately need. Londoners always work together - and together our city will get through the winter and can look forward to better times ahead."
Mr Khan said if London went into further restrictions, the current financial support offered by the government would be "insufficient to keep many businesses and the self-employed afloat".
He warned if the capital moved to tier three, UK Hospitality predicted £2.7bn could be wiped off London's hospitality industry, with 160,000 jobs permanently at risk.
"Theatres and venues in London have begun to reopen, many for the first time since March and after making their venues as safe as possible," he wrote.
"Several major Christmas theatre productions are scheduled in December and January in the West End alone. Last-minute cancellations of these could prove ruinous."
Mr Khan said before any additional restrictions are imposed, ministers must set up a compensation scheme for all lost income during the crucial festive period based on last year's returns.
He added workers required to self-isolate must also receive full pay and not just statutory sick pay.
A spokesman for the Department for Education said: "Schools, colleges and early years settings across the country have worked tremendously hard to put protective measures in place that are helping reduce the risk of the virus being transmitted."
The Regional Schools Commissioner for the South East of England and South London would be continuing discussions with Greenwich, he added.
Last week it was announced all pupils, their families and teachers in parts of London, Kent and Essex should take a Covid test with extra mobile testing units brought in.
It comes as east London and parts of Kent and Essex became some of England's major coronavirus hotspots.
'Blindingly obvious fact'
Three in four boroughs in the city have registered an increase in Covid-19 cases, figures released last week from the Office of National Statistics show.
Kevin Courtney, joint general secretary of the National Education Union, said it supported Greenwich Council and the mayor in their call for schools to move to online learning.
He said: "The government should have been planning for this weeks ago.
"They have now started to recognise the blindingly obvious fact that transmission is happening in schools and that this can spread to families. But the government now needs to act.
"Much more is needed to control the virus in schools and to protect communities."
Parents outside Robert Owen Nursery School and Christ Church Church of England Primary School, both in Greenwich, reacted to the news schools in the borough would be closing early.
One mother said: "I really feel for the kids.
"After a pretty up and down year, a year that hasn't been the best, it would be nice to end the year with a couple of parties but I completely understand and I think the nursery does a great job."
'Too short notice'
Many questioned the timing.
One mother said: "It's 2.5 days, so I don't see what difference this is really going to make and I think the timing of it is really, really bad.
"I'm on maternity at the moment but if I was working, it's just too short notice to get any kind of childcare arrangements in place."
One father added: "I think the timing might be right as a lot of people will be gathering for Christmas and it takes 10 to 14 days to show up so it may be damage limitation.
"I hope it will have an impact. If not, then it's just political."