Anyone who travels to the UK will need to take a PCR test by the end of the second day of arrival.
They must also self-isolate until they get a negative result, the Prime Minister has announced as the country desperately tries to stop the spread of Omicron.
The new variant of Covid spreads very rapidly and can be spread amongst people who are double vaccinated, Boris Johnson warned the public in a press conference after two cases were identified in the UK.
He added that there will be a ‘tightening up' of the use of face coverings in shops and public transport - which had not been mandatory in recent weeks.
"The UK's plan against Covid has been working," he said. "We have had the fastest vaccine roll-out in Europe and now the fastest booster campaign in Europe with almost 16.8 million boosters in people's arms.
"Case numbers have remained relatively high but we are seeing falling numbers of hospitalisataions and falling numbers of deaths.
"But on Wednesday we received news of a new variant, the so-called Omicron variant. I want to express my deep gratitude to scientists in South Africa who identified this new variant and shared this information widely and immediately.”
Health Secretary Sajid Javid said "this is a real reminder that this pandemic is far from over" as he urged people to get their vaccines, including boosters.
"We were concerned from the moment we first identified this new variant," Mr Javid told reporters.
Meanwhile, Green Party leader Eamon Ryan says he was "frightened" when hearing about the new Covid-19 variant which threatens to lock down the world.
He told The Anton Savage Show: “I'll be honest, I was frightened yesterday, it was a blow, I was thinking if we get these boosters out in the next few weeks we'll be in better shape, and we will...and the prospect you'll have something that will work around the vaccine, let's be honest it was the most disheartening thing since the start of this thing”
