Booster jabs will be offered to all over 18s in the UK in response to the new Omicron variant - 11 cases have been detected
The gap between second doses and boosters is also being reduced from six to three months
Severely immunocompromised people will be offered a fourth dose and children aged 12 to 15 will be invited for a second dose
The Scottish and Welsh governments call for tougher travel restrictions but Downing Street rejects this
In the US, Covid adviser Anthony Fauci says it's "not the time to panic" but that spread is inevitable
US President Joe Biden tells Americans: "Don't wait, go get your booster" and urges mask-wearing indoors
Biden says he does not anticipate any further US travel restrictions or lockdowns at this time
And the UN chief, Antonio Guterres, says he is deeply concerned about the isolation of southern Africa over Omicron
Van-Tam also praises the "excellence" of South African scientists - and the transparency of the authorities there - for bringing Omicron to the world's attention at the earliest possible moment.
The variant was first identified there last week.
A bit more from Jonathan Van-Tam.
He says it has always been the case we have known a variant will give us heightened concern at some point.
"We are at that point with Omicron. It is the new kid on the block," he says.
There is no signal from South Africa at the moment, where the variant was first identified, about how severity of disease might be affected with Omicron.
Scientists will be looking at it in more detail over the coming weeks, says the professor.
Some evidence currently shows higher antibody levels will help protect more than lower antibody levels, he adds.
Speaking about the possibility of a new vaccine to target the Omicron virus variant, Raine says that no new vaccine will be approved unless quality and safety standards can be met.
So far, the UK has approved four vaccines for use: Pfizer-BioNTech, Oxford-AstraZeneca, Moderna and Janssen.
More than 50 million people have had a first vaccine dose and more than 46 million have had both doses.
More than 16 million booster doses have been administered across the UK.
Dr June Raine, the chief executive of the MHRA medicines watchdog, says ensuring the continued use of existing vaccines is a priority.
She says since boosters began being rolled out "we have identified no new safety concerns".
"When you're called for your booster dose you can be confident the benefits... outweigh the risks," she says.
Raine says a thorough review of data shows that - for those under-18 - there are no new safety concerns around the offer of a second dose.
The number of mutations on Omicron does give cause for concern, says Van-Tam.
There is an increased growth rate associated with it - but that is not the same as saying an increase in transmissibility, he adds.
"I do not want people to panic at this stage," he says.
It is likely there will be an impact on the effectiveness of vaccines, he says.
But the biggest effect is likely to be in preventing infections - hopefully there will be a smaller effect in preventing disease, he adds.
Nick Triggle
Health Correspondent
Covid booster jabs should be offered to all over 18s to help stop a potential wave driven by the new variant Omicron, the UK government’s vaccine advisers say.
The advice has been given to ministers in all parts of the UK.
The gap between second dose and booster should also be reduced to three months – down from the six-month gap currently being used.
Children aged 12 to 15 should also be invited for a second dose three months after their first.
The rollout of the booster programme should be prioritised so people are offered in order of vulnerability.
The advice has been made by the JCVI.
England's deputy chief medical officer Jonathan Van-Tam is first up at the Downing Street press conference this afternoon.
He has started by talking about the Omicron variant.
He says: "We've known from outset of pandemic, variants have been inevitable."
Prof Van-Tam says this variant is of increased concern - but there is far more we don't know about Omicron yet than we do know.
"Everyone needs to give us time to assemble that data," he says.
He's speaking alongside Covid vaccine adviser Prof Wei Shen Lim and chief medicines regulator Dr June Raine to give an update about who can get a coronavirus booster jab.
It is not long now until we hear from government scientific and medical advisers at Downing Street.
England's deputy chief medical officer Prof Jonathan Van-Tam, Covid vaccine adviser Prof Wei Shen Lim and chief medicines regulator Dr June Raine will speak at 15:00 GMT.
It comes as the UK's vaccine advisory body has been looking at whether it will back an expansion of the Covid booster scheme.
We'll bring you live updates as they happen.
Branwen Jeffreys
Education Editor
Some secondary schools in England brought back masks in corridors after half term, alarmed by the rising cases.
Now all are “strongly advised” to require them from pupils and staff in communal areas.
Education unions have pointed out the virus will make no distinction between a classroom and a corridor. The government says in class pupils are sitting at their desks.
More than 277,000 CO2 monitors out of the 300,000 promised for education settings in England have been delivered.
The under 16s remain the largest unvaccinated group in the population, with an initially patchy roll out of the vaccine offer now picking up pace.
This still means the new highly transmissible Omicron has potential to cause chaos for schools and parents.
That’s because unlike the other variants any close contacts will be required to isolate for 10 days.
Some teachers have become ill with Covid even after having both jabs, although this is hard to quantify.
Image source, Getty ImagesNew measures are coming in because of concerns about the Omicron variant that's just emerged.
There are some UK-wide measures, while others just affect England.
Here's what is changing:
You can read more detail here.
Downing Street has rejected the calls from Scotland First Minister Nicola Sturgeon and Wales FM Mark Drakeford for the self-isolation period for travellers to be extended until the result of a test on day eight after arriving in the UK.
Extending the requirements would have a "detrimental effect" on the travel industry, No 10 said.
The spokesman added: "We believe that the approach we've taken is the proportionate one to the evidence that we currently have available about this variant.
"Introducing further isolation requirements and testing requirements would have a detrimental effect on the travel individually industry and indeed those who are planning to go travelling."
After a request from Sturgeon and Drakeford for a Cobra meeting involving all four nations of the UK, No 10 said there was not one scheduled.
Reality Check
Image source, ReutersA passenger at Johannesburg airport trying to find a flight
South Africa’s President Cyril Ramaphosa has criticised the imposition of travel bans by countries around the world - including the UK - in response to the new Omicron variant.
“The prohibition of travel is not informed by science, nor will it be effective in preventing the spread of this variant.”
The World Health Organization (WHO) urges a “risk-based” approach to travel bans, taking into account other measures like face masks, social distancing as well as the extent of vaccine programmes.
Its advice has shifted significantly from the early days of the pandemic in 2020 when it questioned the value of travel bans. At that time, the WHO said: “Travel measures that significantly interfere with international traffic may only be justified at the beginning of an outbreak, as they may allow countries to gain time”.
Some scientific studies have suggested that these bans only work right at the start of an outbreak combined with other health measures, and that they could risk inducing a false sense of security in populations.
Dr Deepti Gurdasani, an epidemiologist at London’s Queen Mary University, told the BBC that although travel restrictions might slow down the spread of disease, it was clear that the Omicron variant was already in other parts of the world now.
“Rather than travel bans, you should have proper screening and isolation policies which would slow the spread,” she said.
There have now been nine cases of the Omicron coronavirus variant found in the UK so far - with more expected in coming days.
It presents differently to other variants of the virus, which means cases of it can be identified, which helps track its spread.
This graphic below shows how it's detected.
Image source, Getty ImagesMore now from Downing Street, where Boris Johnson's official spokesman has told journalists it is up to employers to decide on the "right balance" for them when it came to whether staff worked from home or the office.
On whether employers were still being encouraged to get people back into the office, the spokesman said: "Our position... has not altered from what it was previously. We [are] obviously keeping the evidence of this variant under review and we will take action if necessary, but currently we don't think there are any other changes required."
He added: "In line with the guidance, we've said that it is safe for people to return to work. It is up to individual employers to decide what is the right balance for [them]."
James Gallagher
Health and science correspondent, BBC News
You might be wondering what’s the point of a booster if the current vaccines turn out to be less effective against Omicron?
We don’t know if that will be the case, but mutations in Omicron mean the jabs may not perform as well.
The vaccines will have taught your body to spot the original coronavirus that emerged in Wuhan in China.
The mutations on Omicron are like it putting on a disguise to slip past your body’s guard.
But boosting is the equivalent of hiring more guards to compensate. You’ll have more crucial components of the immune system such as antibodies and T cells circulating in the body to increase the chances of seeing through the virus’s disguise.
If Omicron does prove to be a problem then boosting is the quickest way of enhancing people‘s protection as it will take around 100 days to update the vaccines if (and it is a major if) that turns out to be necessary.
We are expecting to hear from government scientific and medical advisers later this afternoon.
England's deputy chief medical officer Prof Jonathan Van-Tam, Covid vaccine adviser Prof Wei Shen Lim and chief medicines regulator Dr June Raine will appear at Downing Street from 15:00 GMT.
It comes as the UK's vaccine advisory body has been looking at whether it will back an expansion of the Covid booster scheme.
And after nine cases of the Omicron variant have been found in the UK so far and Number 10 has set out measures to contain it.
Image source, PA MediaThere are no plans from the UK government to introduce the requirement for passengers to take a PCR test on day eight as well as day two on their return, Downing Street has confirmed.
The Scottish and Welsh governments have called for eight-day self-isolation restrictions for all new arrivals to the UK - with tests on day two and day eight - in a letter to Prime Minister Boris Johnson.
The government says the current system of self-isolation until a negative test on day two is a "proportionate one to the evidence we currently have available".
"Introducing further isolation and testing requirements would have a detrimental effect on the travel industry and those planning to go travelling," the prime minister’s spokesman says.
He said the government is not planning to change its approach with regards to furlough either, adding: “Our response needs to be balanced. We are taking a precautionary approach.
"We will keep our measures under review. We believe this is the proportionate and balanced approach."
Image source, Alex ClarkeAlex Clarke found a flight from South Africa - but says UK quarantine hotels are full
With the emergence of the new variant and travel restrictions popping up all over the place, travellers and airlines have been scrambling to rearrange plans.
One passenger told the BBC he had found a return flight from South Africa, but now cannot get a quarantine hotel in the UK.
Alex Clarke, from near Reading, said a lack of hotel space meant he might have to re-arrange his return again.
"I should have landed back in the UK this morning," he told the BBC. "My flight was cancelled, but I managed to book another for Friday. Now that's been cancelled.
"Found return flights via Dubai and Amsterdam, but they have now been closed off. I now have a return booked for Thursday, but the website that books quarantine hotels says everything is booked."
Mr Clarke, who has been staying with friends while his wife and two young children stayed at home in the UK, estimates he faces extra costs of about £4,000.
Read more about how new restrictions are having an impact on travellers and airlines.
Rob Cameron
BBC Prague Correspondent
Back in Europe, a hospital in Liberec, north Bohemia, has confirmed that a Czech woman who came back from Namibia recently does have the Omicron variant of the coronavirus.
The hospital said lab tests had proved the presence of Omicron using sequencing of the virus RNA.
The woman, who is fully vaccinated, is said to be experiencing mild symptoms and is self-isolating.
Prime minister Andrej Babis said on Saturday she had returned from Namibia via South Africa and Dubai.
Reality Check
With the detection of the new Omicron variant, there is concern about relatively low vaccination rates in South Africa.
About 41% of adult South Africans have received a single dose of the vaccine with 35% of adults fully vaccinated, according to official data.
Daily vaccination rates have been dropping and are currently below the UK, EU and other countries.
South Africa has faced a challenge with fewer people turning up for the jabs and is doing fewer than 150,000 vaccinations a day against a target of 300,000.
It recently asked Pfizer to delay vaccine deliveries because of the amount of stock that has built up.
“We are worried [that] this outbreak is largely [among] young people…so this is a very high risk,” Health Minister Joseph Phaahla says.
About 26% of those aged 18 to 35 years have had a single dose, with 21% fully vaccinated.
The Department of Health in South Africa believes vaccine misinformation has played a role.
Some of the anti-vaccination themes seen elsewhere in the world have found a particular resonance in South Africa.
The introduction of vaccine passports in some situations has led to comparisons with the passes that black South Africans had to carry during the apartheid era, even though the passports are for all residents.
Misinformation has also spread about vaccine safety - despite the large volume of evidence from clinical trials and the billions already jabbed showing complications are extremely rare.