Millions more people in England will be placed into lockdown from Boxing Day as ministers acknowledged mounting concern about the spread of coronavirus.
Health Secretary Matt Hancock said the variant coronavirus was spreading at a "dangerous rate" as he announced the fresh Tier 4 restrictions.
He also said cases of another new mutant coronavirus linked to South Africa had been found in the UK and placed travel restrictions on the country.
Areas moving to the toughest Tier 4 – where there is a stay at home order – from Boxing Day are: Sussex, Oxfordshire, Suffolk, Norfolk and Cambridgeshire, those parts of Essex not yet in Tier 4, Waverley in Surrey and Hampshire – including Portsmouth and Southampton but with the exception of the New Forest.
Bristol, Gloucestershire, Somerset including the North Somerset council area, Swindon, the Isle of Wight, the New Forest and Northamptonshire as well as Cheshire and Warrington will all be escalated to Tier 3.
Cornwall and Herefordshire move from Tier 1 to Tier 2.
Mr Hancock told a Downing Street press conference: "Just as we had got a tiered system in place that was able to control this virus we have discovered a new, more contagious virus – a variant that is spreading at a dangerous rate."
The changes mean an additional six million people will be in a Tier 4 lockdown from Boxing Day.
This means a total of 24 million people will now be in Tier 4, or 43% of the population of England.
The Health Secretary set out details on the discovery of the South African variant in the UK.
He said the two cases were contacts of people who have travelled from South Africa over the past few weeks.
"This new variant is highly concerning because it is yet more transmissible and it appears to have mutated further than the new variant that has been discovered in the UK," he said.
The Health Secretary said the cases and their close contacts have been quarantined.
He announced immediate restrictions on travel from South Africa and the Government is telling those who have been to the country in the last fortnight and their close contacts to quarantine immediately.
The law will be changed to make that enforceable, he added.
The challenge facing the Government was underlined by official estimates showing the R value, the average number of people someone with coronavirus infects, has increased to 1.1 to 1.3, up from 1.1 to 1.2 a week ago.
The number of new infections is growing by between 1% and 6% every day, up from 1% to 4% last week.
Professor Neil Ferguson, a member of the Government's New and Emerging Respiratory Virus Threats advisory group (Nervtag), said he expected lockdown measures to help control the spread of the variant.
He told the committee on Wednesday: "Schools are now shut, we are in a near-lockdown situation across the country, contact rates are lower over Christmas.
"I expect, though I hesitate to make any sort of predictions, we will see a flattening of the curve in the next two weeks. We will see at least a slowing of growth."
Meanwhile, tempers flared at Dover as the French reopened the border to allow lorries to cross the Channel for the first time since emergency restrictions were imposed in response to the new variant.
More than 5,000 HGVs are being held in Kent, with drivers being tested for Covid-19 before they are allowed into France.
There were reports of disturbances at Dover and at the lorry holding facility in Manston involving those waiting to cross the Channel.
One man was arrested for obstructing a highway in Dover.