LONDON -- Two cases of the Omicron coronavirus variant have been detected in Britain, British Health Secretary Sajid Javid said Saturday.

He said the cases, detected in Chelmsford and in Nottingham, are "linked" and the two individuals are now self-isolating along with their households while further tests and contact tracing take place.

In a press conference at Downing Street on Saturday, British Prime Minister Boris Johnson said all travellers entering Britain must take a PCR test by the end of the second day after their arrival, and must self-isolate until they receive a negative test result.

Meanwhile, rules on face coverings will be tightened up in shops and public transport, he said, adding that these measures are "temporary and precautionary" and will be reviewed in three weeks.

Downing Street have subsequently tweeted: "Face coverings will become compulsory on public transport and in shops. But not including hospitality."

"There are many things that we just cannot know at this early stage," Johnson said, noting that it does appear Omicron spreads very rapidly.

He warned that the "extensive mutation...might at least in part reduce the effectiveness of our vaccines".

The "targeted measures now" will "buy time for our scientists" to know what they are "dealing with", as well as facilitating more booster jabs in arms and keeping pressure off the National Health Service (NHS), he said.

Four more countries have been added to Britain's travel "red list" effective from 0400 GMT on Sunday: Angola, Mozambique, Malawi and Zambia.

The British government has previously added South Africa, Botswana, Lesotho, Eswatini, Zimbabwe and Namibia to the country's travel red list. Flights from these nations to Britain have been suspended until 0400 GMT on Sunday when hotel quarantine is up and running.

Passengers arriving in Britain from these countries from 0400 GMT on Sunday will be required to book and pay for a government-approved hotel to quarantine for 10 days. The British government has urged anyone who has arrived from these countries recently to get tested.

Omicron, also called B.1.1.529, has also been identified in South Africa, Botswana, Belgium, Israel and possibly in other countries and regions.

A growing number of countries and regions are suspending flights from southern African countries due to the new variant, which has sparked serious concern among scientists and researchers and triggered a massive slump of stocks in Asia, Europe and the United States.

The World Health Organization (WHO) declared on Friday the latest variant B.1.1.529 of SARS-CoV-2 to be "of concern", its most serious level, and officially gave it the Greek name Omicron. The WHO has asked countries to enhance surveillance and sequencing efforts to fight the new variant.

Scientists are still unclear whether existing antibodies would react well to Omicron, which has 32 spike protein mutations, more than previously found variants.

The country reported another 39,567 coronavirus cases in the latest 24-hour period, bringing the total number of coronavirus cases in the country to 10,110,408.

Britain also recorded another 131 coronavirus-related deaths. The total number of coronavirus-related deaths in Britain now stands at 144,724. These figures only include the deaths of people who died within 28 days of their first positive test.

More than 88 percent of people aged 12 and over in Britain have had their first dose of vaccine and more than 80 percent have received both doses, according to the latest figures. More than 29 percent have received booster jabs, or the third dose of a coronavirus vaccine.

To bring life back to normal, countries such as Britain, China, Germany, Russia and the United States have been racing against time to roll out coronavirus vaccines.