Video report by ITV News Political Correspondent Paul Brand
Matt Hancock said a new, fast-moving variant of Covid-19 had recently been identified, which "may be associated with the faster spread in the South of England."
He said all 32 boroughs of Greater London and the City of London, along with parts of Essex and Hertfordshire, will move into the highest tier of Covid-19 restrictions from 00:01 on Wednesday.
The Cabinet minister, in an update to MPs, said initial analysis of the new strain shows it is "growing faster than the existing variants", but is no more deadly.
And there is no evidence to suggest the new variant impacts the effectiveness of coronavirus vaccines.
The health secretary told MPs: “I must stress at this point that there is currently nothing to suggest that this variant is more likely to cause serious disease.
"And the latest clinical advice is that it’s highly unlikely that this mutation would fail to respond to a vaccine, but it shows we’ve got to be vigilant and follow the rules and everyone needs to take personal responsibility not to spread this virus.”
He said over 1,000 cases of the new strain had been identified, predominantly in the South of England but also in nearly 60 different local authority areas.
"And numbers are increasing rapidly," he added.
It is being assessed at Porton Down and its potential impact will be known in the "coming days and weeks".
On the move to tighten restrictions in much of south England, Mr Hancock said the decision was taken early, before the planned review date on Wednesday due to a significant rise in cases.
It is understood the restrictions will be reviewed on December 23.
The Department for Health said the situation in London, parts of Essex and South Hertfordshire has "deteriorated" since the end of England's national lockdown.
Mr Hancock told MPs that cases are doubling every seven days in the capital.
In Essex, the following areas will go into Tier 3:
- Braintree- Basildon - Brentwood - Chelmsford - Epping Forest - Castle Point - Harlow - Maldon - Rochford - Thurrock - Southend-on-Sea
- Watford - Three Rivers - Broxbourne - Herstmere
Watch Matt Hancock's update to MPs in full:
The rule change will see more than 10 million people placed into the highest tier of coronavirus restrictions.
It comes after London MPs were told by health officials earlier that the capital is moving towards a phase of “exponential growth” in its number of coronavirus cases.
It follows warnings from London Mayor Sadiq Khan that Tier 3 restrictions - which will involve the closure of pubs and restaurants except for takeaway and delivery services - would have a "catastrophic" economic impact.
On Thursday, it was revealed London has the highest rates of coronavirus in England, with the weekly case rate at 225 per 100,000.The borough of Havering has the highest coronavirus rate in London, at 471 per 100,000.In South Essex, case rates are greater than 200 per 100,000 in 10 out of 14 local authorities and are increasing across the region, with cases in Basildon reaching 563 per 100,000.
In South Hertfordshire case rates are greater than 150 per 100,000 in 4 out of the 5 local authorities bordering London and have increased by 10% or more over the last 7 days.
Broxbourne’s cases have reached 358 per 100,000.
The Department for Health said there would be a rise in hospital admissions and deaths if action was not taken.
The government is resisting pressure to close schools in the capital, despite evidence showing the spike in the capital's cases has mainly come from young people.
Mayor Khan said he is "extremely worried" that a rise in cases among young people could result in them "inadvertently take the virus back home and give it to granny next week".
"Unless they can make schools Covid safe, those schools that aren't Covid safe, haven't got the testing, should probably close a few days early before Christmas, and open a bit later, while they sort out the testing."
His suggestion, which was set out in a letter to the PM, was swiftly rejected by Downing Street, with Mr Johnson's official spokesman saying the government expects "all schools and colleges to remain open until the end of term on Thursday".
What can you do in each tier of England's coronavirus restrictions?
Tier 1: Up to six people can meet indoors or outdoors. Pubs and restaurants can open, with last orders at 10pm and closing at 11pm.
Tier 2: No mixing indoors, apart from support bubbles. Up to six people can meet outdoors. Pubs and restaurants can open, with last orders at 10pm and closing at 11pm - but alcohol can only be served with a substantial meal.
Tier 3: No mixing indoors. People can only meet outdoors in limited places such as parks and public gardens. Pubs and restaurants must close, with the exception of takeaway sales.
In all tiers, non-essential shops, gyms, hairdressers and personal care services can open. People in all tiers to work from home where they can do so. Full details on what you can do in each tier here