Boris Johnson has said that people who come into close contact with anyone who tests positive for coronavirus with the new Omicron variant will have to self-isolate, regardless of their jab status, as he delivered a press conference in Downing Street.
The prime minister warned the Omicron variant could reduce the effectiveness of vaccines, as he announced a strengthening of England's rules, after two infections of the new strain were identified in Nottingham and Brentwood in Essex.
In an attempt to slow the spread, Mr Johnson announced "temporary and precautionary" measures to be reviewed in three weeks, alongside a "boost" to the booster vaccination campaign.
Mr Johnson said that day two PCR tests are back for all international travellers, saying: "Will require anyone who enters the UK to take a PCR test by the end of the second day after their arrival and to self-isolate until they have a negative result".
There will also be a ramping up of mask wearing.
It comes after the UK Health Security Agency confirmed the two new cases in the UK, 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 will take place in the areas where the cases were found.
Malawi, Mozambique, Zambia and Angola will face travel restrictions from Sunday, when they will join South Africa and five other neighbouring nations on England's red list.
Professor Chris Whitty added it is "inevitable" the Omicron variant will spread across the world over the next few days.
England's chief medical officer said while it is clear Omicron is highly transmissible, it is not yet clear how effective the vaccine will be as protection against it.
However, he said those who are vaccinated or receive the booster jab will be less likely to become seriously ill.
The UK's chief scientific adviser Sir Patrick Vallance also warned the UK may need to "face up" to the possibility of further action if the Omicron variant is very transmissible.
Sir Patrick told the Downing Street press conference: "I think we'll get more information on transmissibility, we'll get more information on the ability of the vaccines to protect against the virus, but that's going to take a little bit of time.
"At the moment, the models are more 'if it spreads very fast, of course it's going to spread very fast and go into a lot of places, and if it spreads less fast it's going to do so less'.
"But if it's very transmissible and does cause big escape, then clearly that's a major issue we have to face up to.
"But that isn't what we know at the moment, we need to get that information."