At a hastily arranged press conference, Boris Johnson announced a raft of new restrictions to try to tackle the spread of the new variant even though scientists have suggested that it is not as problematic as the Delta variant last year. Among the new restrictions will be compulsory mask-wearing in shops and on public transport but not in bars and restaurants.
A raft of new countries were added to the red list but anybody arriving from anywhere abroad will now have to self isolate until they have had a negative result on a PCR test with the option of lateral flow tests dropped.
In addition anybody who has been in contact with a covid case will have to self isolate even if they have been vaccinated.
The Prime Minister has also asked the independent JCVI to review the current vaccine programme to see if children under 12 can have the jab and whether booster jabs can be administered after five months instead of the current six month period.
The Government has however resisted bringing in so-called vaccine passports for pubs and public events.
It was revealed earlier today that two cases of the Omicron variant had been detected in Nottingham and Brentwood, Essex, as the Government added four more southern African nations to the travel red list.
The UK Health Security Agency confirmed the cases, which are both believed to be connected and linked to travel to southern Africa, after genomic sequencing overnight.
The individuals and their households were ordered into self-isolation and targeted testing was being carried out in areas where they are thought to have been infectious.
Health Secretary Sajid Javid said "this is a real reminder that this pandemic is far from over".
Mr Javid added: "We were concerned from the moment we first identified this new variant.
"It's a deeply concerning new variant and we do need to learn more about it but the fact that we now have these two cases in the United Kingdom does mean we need to take further measures and that's why I've set this out today."
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Addressing the nation alongside chief scientific adviser Sir Patrick Vallance and chief medical officer Professor Chris Whitty, Mr Johnson opened the press conference by saying: "The UK's plan against Covid has been working.
"But on Wednesday we received news of a new variant, the so-called Omicron variant.
"This variant is spreading around the world with two cases so far in the UK.
"As always with a new variant there are many things we just cannot know at this early stage.
"And it does appear that Omicron spreads very rapidly and can be spread between people who are double vaccinated."
He added: "So we need to take targeting and proportionate measures now as a precaution while we find out more.
"First we need to slow down the seeding of this variant in our country.
"We need to buy time for our scientists, and for us to get more people vaccinated and boosted, as well as to help our NHS prepare for what is already a challenging winter."
Prof Whitty added: "Spread is clear and it's spreading around the world."
The measures will be reviewed in three weeks.
Mr Johnson said: "Right now this is the responsible course of action."