Matt Hancock to lead Downing Street press conference tonight on coronavirus.
The Health Secretary is expected to give an update to MPs at 3.30pm in the Commons amid growing speculation London could be plunged into tougher restrictions.
MPs were told cases are “off the chart” at briefing on the scale of the virus the capital on Monday.
London is currently in Tier 2 but several MPs said they were left in no doubt by officials that the capital will move to Tier 3.
The tiers are due to be reviewed on Wednesday but a decision could be made sooner.
London Mayor Sadiq Khan called for schools to shut early for Christmas and reopen later in January to curb the spread of the virus.
Secondary school-age children have been identified as a group of particular concern in London, prompting Mr Hancock to announce plans to send testing units to schools.
Mr Khan called for more financial support for the capital’s businesses, arguing that the rise in cases was not linked to hospitality venues but instead due to a significant increase in schools and colleges.
He said the decision could come as early as Monday.
The mayor told Sky News: “My understanding is that Covid-O is meeting as we speak – that’s the sub-committee of the Cabinet that makes the recommendations.
“We will have to wait and see what the Government decides – it’s a Government decision, not my decision or London leaders’ decision.”
He said “it’s possible” that a decision will be made today “because we have seen over the last few days a big increase in the virus.”
But he added: “It’s worth looking where the virus is spreading faster and it’s worth looking at the areas where we have seen the biggest increase.
“It’s worth them asking themself the question whether a move to Tier 3 is a blunt instrument that doesn’t really address in a laser-like fashion where we are seeing the biggest problems.”
In his letter to the Prime Minister, the Mayor said the seven-day case rate had risen in 32 local authority areas in the capital compared to the previous week.
There were also 17 boroughs where the seven-day rate exceeded 200 cases per 100,000 people.
According to the latest figures, the borough of Havering has the highest coronavirus rate in London, with 1,314 new cases recorded in the seven days to December 9 – the equivalent of 506.3 cases per 100,000 people.
This is up from a rate of 321.3 in the seven days to December 2.