I have just heard that Boris is going to talk on TV about the South East today. Don't know what he is going to do as we are in Tier3.
Covid: Boris Johnson 'to tighten rules' in London and south-east England
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A fourth tier of coronavirus restrictions is expected to be introduced in London and south-east England, sources have told the BBC.
There is also expected to be a tightening of the plans to relax the rules around households gathering during the Christmas period.
The PM is to hold a press conference at 16:00 GMT as concerns grow about the spread of a new variant of the virus.
Boris Johnson hosted a cabinet meeting earlier to discuss what action to take.
Chief medical officer Prof Chris Whitty said government advisers "consider the new strain can spread more quickly".
He will join the prime minister and Sir Patrick Vallance, the government's chief scientific adviser, at the press conference.
England's current three-tier system of coronavirus measures has faced criticism for not being effective in stopping the surge in cases.
The first ministers from Scotland, Wales and Northern Ireland held talks with Cabinet Office minister Michael Gove earlier.
Scotland's Nicola Sturgeon, who will hold a press conference later,
tweeted: "Cases currently at lower level in Scotland than UK - but preventative action may be necessary to stop faster spreading strain taking hold."
Welsh ministers are meeting to discuss "serious concerns", while Northern Ireland First Minister Arlene Foster
tweeted that the new variant of coronavirus prevalent in south-east England was "very concerning".
Wales and Northern Ireland have already announced post-Christmas lockdowns. Covid rules were due to be relaxed across the UK between 23 and 27 December, with up to three households being able to meet.
Any change of restrictions for London and the South East would not require MPs' approval in a Commons vote, as they are only held for England or UK-wide measures.
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It was just 72 hours ago that Boris Johnson was resisting pressure to cancel relaxations of restrictions over Christmas. He told a press conference cancelling the festive plans would be "inhuman".
But concern has grown significantly in the last couple of days; both about the rising number of cases and the new variant of the virus disclosed earlier this week.
Ministers met with their scientific advisers on Friday evening - and again on Saturday morning. Things have moved quickly in the last 24 hours.
Sources have confirmed a new tier 4 will be introduced in London and the South East of England. That's likely to mean closure of non-essential shops and beauty salons.
That's likely to have a significant impact of Christmas plans and what people can do in and around the capital.
There are also conversations taking place in the devolved administrations about whether further restrictions are needed there too.
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Following concern about the rapid spread of a new variant, Prof Whitty said: "We have alerted the World Health Organization and are continuing to analyse the available data to improve our understanding."
That came after advice from the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag) - the expert committee that advises the government on pandemics.
"There is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments although urgent work is under way to confirm this," Prof Whitty added.
Chairman of Nervtag, Peter Horby, said they had not found the variant to be more severe but being easier to transmit meant it was harder to control.
New variant 'really worrying'
Paul Hunter, professor of medicine at the University of East Anglia, said it was a big concern that the new variant seemed to be "out-competing the other viruses very fast indeed".
He told the BBC News Channel: "The really worrying thing from my perspective is that this was something we didn't know about at the beginning of this month and it appeared. And from what we hear, it is already becoming one of the dominant strains."
Asked if the transmission would move northwards, he said "almost certainly".