The planned relaxation of Covid rules for Christmas has been scrapped for giant parts of south-east England and move just Christmas for the remainder of England, Scotland and Wales.
From midnight, a replacement tier four are going to be introduced in areas including London, Kent, Essex and Bedfordshire.
Elsewhere in England, Scotland and Wales, relaxed indoor mixing rules are cut from five days to Christmas .
Those in tier four cannot mix indoors with anyone not from their household.
Prime Minister Boris Johnson announced the changes for England at a Downing Street briefing after scientists said a replacement coronavirus variant is spreading sooner .
Tier-four restrictions – almost like England’s second national lockdown – will apply altogether areas within the South East currently in tier three, covering Kent, Buckinghamshire, Berkshire, Surrey (excluding Waverley), Gosport, Havant, Portsmouth, Rother and Hastings.
It will also apply in London (all 32 boroughs and therefore the refore the City of London) and the East of England (Bedford, Central Bedford, Milton Keynes, Luton, Peterborough, Hertfordshire, Essex (excluding Colchester, Uttlesford and Tendring).
In Wales, First Minister Mark Drakeford announced that the country are going to be placed under lockdown from midnight.
Tier four graphic
For tier-four areas in England, a stay-at-home order has been issued, with exemptions for those that need to visit work or for education.
Social mixing are going to be move meeting one person in an open public space.
All non-essential retail will need to close, along side hairdressers, nail bars, indoor gyms and leisure facilities.
People are going to be advised to not travel into a tier four area.
The restrictions will last for 2 weeks, with the primary review due on 30 December.
Mr Johnson told the Downing Street briefing: “I skills disappointing this may be, but we’ve said throughout this pandemic that we must and that we are going to be guided by the science.
“When the science changes, we must change our response. When the virus changes its methods of attack we must change our method of defence and as your prime minister I sincerely believe there’s no alternative hospitable me.”
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Analysis box by Nick Triggle, health correspondent
The steep increase within the proportion of coronavirus cases linked to the present new variant is robust evidence that it’s driving transmission.
In London, 28% of cases were as a results of this new mutation in mid-November, but that has now increased to quite 60%.
It may explain why, during the second lockdown, cases began to increase in London, while in Kent the tier three measures appear to possess had little impact in recent weeks.
As England’s chief medical adviser Prof Chris Whitty says, this is often a nasty moment, but there’s also some hope.
Mutations happen all the time – there are thousands of variations to the present coronavirus since it emerged – and there’s nothing to suggest this causes more serious illness or will hamper the effectiveness of the vaccine.
The prime minister announced 350,000 people had been given the primary dose of the vaccine within the first fortnight of the programme.
In the coming weeks, the amount of GP-led vaccination clinics should increase six-fold, while approval of a second vaccine made by Oxford University could pave the way for mass vaccination centres to be found out in sports stadiums and conference centres.
That could see two million people every week being vaccinated. Within a matter of months all the over-65s could are offered a jab. this might then start to feel very different.
But for now, the slog of the pandemic continues – and for several it just got harder.
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Mr Johnson said the new restrictions were necessary due to the spread of a variant of coronavirus, which was transmitting faster than the first .
He said analysis from New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) suggests the new variant could increase the R number – or reproductive rate of the virus – by 0.4 or more.
Although there’s “considerable uncertainty”, it’s going to be up to 70% more transmissible then the old variant, he said.
Prof Chris Whitty said that while the new strain of coronavirus will make things much worse, if the vaccine works against it there was room for optimism.
He added: “I think this is often a situation which goes to form things tons worse, but there are some really optimistic things if you look once we get the vaccine out, assuming the vaccine works against this, which at the instant is that the working assumption.”
Mr Johnson revealed that 350,000 people within the UK have thus far received their first dose of the Covid-19 vaccine.
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