Two cases of the Omicron variant of the Covid-19 virus have been detected in the UK.
UK Health Secretary Sajid Javid confirmed that the two cases are thought to be connected, with both individuals now self-isolating in Chelmsford, Essex, and Nottingham.
Both of the cases of the new strain, which is feared to have higher re-infection rates, have been linked to travel to southern Africa, after genomic sequencing overnight.
The UK Government has also added four extra African nations to the travel red list in the wake of the news. Malawi, Mozambique, Zambia and Angola have now been added, alongside South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia, meaning anyone who has travelled in the last 10 days to any of these 10 countries must self-isolate and take PCR tests.
Mr Javid ordered targeted testing in the affected areas and said Prime Minister Boris Johnson will "set out further measures" during a Downing Street press conference later on Saturday.
"We were concerned from the moment we first identified this new variant," Mr Javid told reporters.
"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."
Mr Javid did not say whether further restrictions could be added ahead of Christmas in light of the new strain, which the World Health Organisation (WHO) has designated a "variant of concern".
Instead, he said: "We've made a lot of progress, we all want to see that protected and if anyone's sitting at home thinking what can I do? Get vaccinated."
Mr Javid said anyone who has travelled in the last 10 days to the 10 countries now on the red list, with Malawi, Mozambique, Zambia and Angola being added to South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia, they must self-isolate and take PCR tests.
England's chief medical officer Professor Chris Whitty said: "We will continue to work closely with the international community to quickly gather and analyse information on this variant to understand any possible increase in transmissibility or resistance to vaccines."
The UK is the second European nation to have reported the presence of Omicron after Belgium said it had identified a single case on Friday.
Ministers said non-UK and Irish residents who have been in Malawi, Mozambique, Zambia and Angola in the previous 10 days will be refused entry into England from 4am on Sunday.
Those who are permitted to return will be ordered to isolate in a Government-approved facility for 10 days.
South Africa, Botswana, Eswatini, Lesotho, Namibia and Zimbabwe were added to the red list on Friday, and the UK imposed a ban on flights leaving from those nations.
