Changes expected to the UK's Christmas plans as PM holds emergency talks
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The move comes amid reports that the Prime Minister is set to announce a new Tier 4, with tighter restrictions for London and the South East, while cutting back on the planned relaxation of the rules over Christmas.
The chief medical officer for England Professor Chris Whitty said the UK had now informed the World Health Organisation that it had concluded the mutant strain could spread more quickly.
"As announced on Monday, the UK has identified a new variant of Covid-19 through Public Health England's genomic surveillance," he said in a statement.
"As a result of the rapid spread of the new variant, preliminary modelling data and rapidly rising incidence rates in the South East, the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) now consider that the new strain can spread more quickly.
"We have alerted the World Health Organisation and are continuing to analyse the available data to improve our understanding.
"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.
"Given this latest development it is now more vital than ever that the public continue to take action in their area to reduce transmission."
Following the lunchtime conference call with ministers, Mr Johnson will hold a press conference with Prof Whitty and the Government's chief scientific adviser, Sir Patrick Vallance at 4pm.
It is the second meeting in less than 24 hours that the Prime Minister has held on the issue - underlining the strength of concerns around the new variant.
In Scotland, First Minister Nicola Sturgeon said she would be chairing a meeting of her cabinet to discuss the emerging evidence on the new variant this afternoon.