The government has brought some areas into the highest alert level early as a result of recent "exponential" rises in coronavirus cases across parts of England.
London and large parts of Essex and Hertfordshire will move into tier three from Wednesday.
Officials are due to meet on the same day to review the tiers in other parts of the country based on data from last week.
What can the figures tell us about the current situation?
Why were areas moved into tier three early?
When deciding on the initial tiers on 26 November, the government looked closely at the number of cases per 100,000 people - also called the infection rate - across broader county and city regions.
Officials also looked at the rates in over-60s, as this age group is more at risk of getting seriously ill from coronavirus.
Over the past few weeks, London, as well as large parts of neighbouring Essex and Hertfordshire have seen "very sharp exponential rises in the virus" across all age groups, Health Secretary Matt Hancock said.
He said the spread in these areas could be being driven by a "new variant" of the virus.
Since mid-November, London's coronavirus infection rate has risen by nearly a third, from 186 cases per 100,000 on 19 November, to 241 per 100,000 in the week to 9 December.
The rate in Essex increased by more than three quarters over the same period, from 157 cases per 100,000 to 292.
When announcing the early entries to tier three, Mr Hancock said tougher restrictions were needed to protect the NHS.
"Our NHS is straining every sinew to cope with the pressures... but if cases continue to double even they will be overwhelmed," he said.
Where are infection rates falling?
In contrast to areas in south-east England, such as Kent and Essex, many places in tier three in the north of England and the Midlands have seen a significant fall in rates over the past three weeks.
This reflects a broader trend among many tier three areas, which are now far closer in terms of infection rate to those in tier two than before.
The Humber area, making up Hull, East Lincolnshire, East Riding of Yorkshire and North Lincolnshire, saw its rate fall by nearly two thirds from 431 cases per 100,000 people to 150 between mid-November and now.
Similarly the Bristol area saw its rate fall by three fifths over the same period from 324 cases per 100,000 to 128.
However, it is uncertain whether falls in these areas will signal a relaxing of coronavirus restrictions before the Christmas period.
Recently NHS Providers - which represents hospital trusts in England - warned that relaxing Covid rules now could trigger a third wave of the virus during the busiest time of year for hospitals.
Reporting by: Christine Jeavans, Wesley Stephenson, Robert Cuffe, Rob England and Daniel Dunford.