The areas join tier three at 00:01 GMT on Wednesday, meaning some 34 million people will face the toughest rules.

A new variant of coronavirus has been identified that "may be associated" with the faster spread in southern England, Mr Hancock added.

Pubs and restaurants in tier three must close except for takeaway and delivery.

Also under the rules, sports fans cannot attend events in stadiums, and indoor entertainment venues - such as theatres, bowling alleys and cinemas - must remain shut.

The health secretary told a Downing Street briefing that action had to be taken immediately - before the next scheduled review of England's three-tier system on Wednesday - to slow "sharp, exponential rises" in infection, adding that in some areas the virus was doubling around every seven days.

Speaking alongside England's chief medical officer Prof Chris Whitty and Prof Kevin Fenton, London regional director for Public Health England, Mr Hancock warned that hospitals across the capital and in Essex and Kent were already "under pressure".

He described the rise in transmission and the new variant as "a salutary warning for the whole country".

"This isn't over yet," he said.

With Kent, Medway and Slough already under tier three rules, it means large parts of south-east England will soon join much of the Midlands, north-west England and north-east England under the strictest curbs on social contacts.

More than 34 million people in England will be in tier three when the changes come into effect: 21.5 million in tier two; and about 700,000 in tier one.

The latest tier three areas include:

*  Greater London

*  The south and west of Essex (Basildon, Brentwood, Harlow, Epping Forest, Castle Point, Rochford, Maldon, Braintree and Chelmsford, along with Thurrock and Southend-On-Sea borough councils)

*  The south of Hertfordshire (Broxbourne, Hertsmere, Watford and the Three Rivers local authority)

The move comes as another 20,263 cases have been recorded and a further 232 people have died in the UK, the government said on Monday.

What are tier three (very high) rules?

*  You cannot mix indoors, in private gardens or in most outdoor venues, except with your household or bubble

*  You can meet in a group of up to six in other outdoor spaces, such as parks, beaches or countryside

*  Shops, gyms and personal care services (such as hairdressing) can stay open

*  Bars, pubs, cafes and restaurants must stay closed, except for delivery and takeaway

*  Sports fans cannot attend events in stadiums

*  Indoor entertainment venues - such as theatres, bowling alleys and cinemas - must stay closed

*  People are advised not to travel to and from tier three areas

Scientists are keeping a close watch on this variant as there are mutations to the "spike protein".

This is the crucial part of the coronavirus that acts like the key to unlock the doorway into our body's cells.

One mutation is on the crucial part of the spike that makes first contact with the outside of our cells.

Another looks the same as one that emerged separately in mink, but seemed to make it harder for antibodies from Covid survivors to attack the virus.

The three leading vaccines train the body to attack the spike, which is why people are asking if the variant will make the vaccines less effective.

But vaccines teach the immune system to attack many parts of the spike so the view from health officials is these mutations are unlikely to affect the vaccine's effectiveness.

And my golden rule for judging all "new variant" or "new strain" stories remains: Has the virus' behaviour actually changed?

Viruses mutate all the time and they can become more common by just being in the right place at the right time.

We still have no evidence the virus spreads more easily, is more dangerous or would affect the vaccine.

Prof Whitty warned cases in some areas could rise "very rapidly" by Christmas - without action to combat the current doubling rate.

But he stressed it was "possible to turn this around with the tools we have", highlighting how areas such as Liverpool have successfully brought infection rates down.

Prof Fenton said it was a "pivotal moment" for London and parts of south-east England, which was why the government had had to take "quick and decisive action".

Asked whether the government should rethink plans to ease some restrictions for Christmas, Mr Hancock said it was important "everyone is cautious" ahead of the festive period, especially when meeting vulnerable people.

"But we do understand why people do want to get together with their families," he said.

At Christmas, people will be allowed to mix with a slightly wider circle of family and friends, but other tier rules will still apply.

Earlier, Mr Hancock told MPs there was currently nothing to suggest that the new virus variant was more likely to cause serious disease.

His advice was that it was "highly unlikely" the mutation would fail to respond to a vaccine.

Labour's shadow health secretary Jonathan Ashworth warned the country was "heading into the Christmas easing with diminishing headroom".

And while London Mayor Sadiq Khan described the announcement as "incredibly disappointing" for businesses, he said it was clear the virus was accelerating "in the wrong direction" and urged all Londoners to follow the new rules.

Essex County Council leader David Finch said local leaders would lobby the government for support "during this very challenging time", and Hertfordshire County Council's leader, David Williams, called for residents to "stay disciplined and stick to the guidance".

There was anger from London's hospitality and entertainment sectors, who will be hit hard by the fresh restrictions.

Andy Jones, who owns Jones & Sons restaurant in Dalston, east London, said it was "unacceptable to give the hospitality industry 24 hours' notice".

"This week we had about £42,000 worth of bookings in the system already," he said, adding that the restaurant would be closed for business - apart from delivering takeaway meals on Sundays.

"My biggest gripe is that busy restaurants plan a week ahead. We've got a week's worth of food pretty much, either prepped ready to go, bought in already, which you can't send back, or on order."

Theatre producer Sir Cameron Mackintosh - currently putting on the Les Misérables musical in the West End - described the rules as "devastating". He said it made theatres' efforts to ensure safe performances "seem worthless".

Kate Nicholls, of trade body UKHospitality, said the move placed an "unfair, illogical and disproportionate burden on hospitality businesses without effectively tackling Covid".

And Emma McClarkin, chief executive of the British Beer & Pub Association, said the move into tier three was "another nail in the coffin for London's pubs, as well as those affected in parts of Hertfordshire and Essex.

There was also disappointment that sporting events in London would no longer be able to welcome fans from Wednesday.

Despite coronavirus cases continuing to rise, No 10 said the government "has no plans to review the Christmas guidelines" which effectively suspend the tier system of restrictions and allow more households to mix.

In Scotland, people have been told to "cut down" the number of contacts in the week before Christmas if they plan to meet up with relatives.

In Wales, a second health board has suspended non-urgent care amid a continued rise in cases. And a leading doctor in Northern Ireland said its health service is facing a "nightmare" scenario in January.