The Prime Minister has stepped up Covid rules after the new variant was identified in Brentwood, Essex.
It's the first of two cases to be discovered in the UK, with the other in Nottingham.
Sajid Javid confirmed that the two cases are related with a connection to travel in southern Africa - where the variant was first discovered.
The new strain has been named Omicron and has been designated a "variant of concern" by the World Health Organization (WHO).
The Department of Health & Social Care said the individuals that have tested positive, and all members of their households, are being re-tested and told to self-isolate while further testing and contact tracing is under way.
Targeted testing is now being carried out across Essex, as six African countries are added to the red list.
Anyone who travels abroad must take a PCR test on day two of their arrival back in the UK and self-isolate until they get a negative result.
The prime minister also revealed in the update that anyone who comes into contact with someone who has tested positive for the Omicron variant must isolate at home for 10 days, regardless of their vaccination status.
He added that face coverings on public transport and in shops will return, and the booster vaccine rollout will be stepped up to cover more people and the gap between second and third doses will be shortened.
Mr Johnson said the new measures will be assessed in three weeks' time.
When questioned if it meant that Christmas plans could be curtailed for a second year running, Mr Johnson said he is “absolutely confident that this Christmas will be better than last Christmas”, suggesting he has no current plans to introduce a lockdown.
Professor Paul Hunter, from the University of East Anglia, said vulnerable people in poorer nations should have been vaccinated sooner - ahead of those at less risk.
He said this could be one of the reasons why the variant has developed more quickly.
Prof Hunter believes events in the run up to Christmas could be affected, but says right now, he doesn't believe that Christmas will be cancelled.
Boris Johnson warned the new variant could reduce the effectiveness of vaccines.
Mr Johnson said: "We need to take targeted and proportionate measures now as a precaution while we find out more.
"First we need to slow down the seeding of the variant in this country, we need to buy time for our scientists to understand exactly what we're dealing with, and for us to get more people vaccinated, and above all to get more people boosted."
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.
“It is important that everyone takes sensible precautions – get a PCR test if you have symptoms, isolate when asked, wear a face covering in crowded and enclosed spaces, ventilate rooms, get your vaccine and boosters as soon as you can.”