The mutation now accounts for more than 50 per cent of cases in the capital, according to initial analysis, taking over from Delta.
The number of coronavirus patients in London’s hospitals has also jumped to 1,360, the highest since early March when the city was coming out of the second wave.
The majority of them are understood to be not fully vaccinated, with nearly all the cases Delta so far.
While Omicron may be less severe for many people, especially the vaccinated, infections of this variant were estimated to have spiralled to 200,000 a day across Britain.
Amid the unprecedented fast spread, people were queuing for hours to get boosted at vaccination centres in London, including at St Thomas’ hospital and the Centre Court Shopping Centre in Wimbledon.
However, up to two million Londoners have not had any vaccine, leaving themselves vulnerable to the virus and the NHS to being overwhelmed which would impact on healthcare for millions of other people.
London’s public health chief Professor Kevin Fenton told The Standard: “Our latest monitoring of provisional data indicates that over 50 per cent of cases sent for further analysis in London are now Omicron, replacing Delta as the dominant variant.
“It is crucially important that Londoners get fully vaccinated with their first dose, second dose, and the all-important booster while we learn more about the clinical characteristics of Omicron and the potential impact on our hospitals.
“I want to thank Londoners for their immense response over the past couple of days in coming forward for their vaccines and protecting themselves and their communities. As a city we can do this.”
Appealing to the unvaccinated, Jeremy Hunt, chairman of the influential Commons health and social care committee, warned: “Don’t risk your life by not getting vaccinated.
‘People who have had the vaccine are ten times less likely to die from Covid.”
Sir John Bell, Regius Professor of Medicine at Oxford University, added: “The most important thing we could do is to get those people who have not been vaccinated vaccinated, that’s the thing that really grinds the system down.”
He also stressed that the incidence of severe disease, death and hospitalisation into intensive care had “dropped off a cliff” for the double-jabbed.
In other key developments:
A major study from South Africa suggested two doses of Pfizer-BioNTech’s vaccine appeared to give 70 per cent protection against hospitalisation, and 33 per cent protection against infection.
Children appeared to have a 20 per cent higher risk of hospital admission with complications during the Omicron-fuelled wave than during the first, though this was still a small number of cases.
Dr Angelique Coetzee, chair of the South African Medical Association who was one of the first clinicians to detect Omicron, told the Commons science and technology committee, that the country has so far largely seen “mild disease” in patients. However, we see a different picture in the hospital admissions, especially around unvaccinated people, of which the majority are unvaccinated,” she added.
As the Government in the UK seeks to get 20 million people jabbed by the New Year in the fastest roll-out ever, Deputy Prime Minister Dominic Raab acknowledged “teething problems” after people queued for hours to get jabbed.
Home lateral flow test kits were not available for a second day running on the Government website due to problems sending them out.
Professor Martin Marshall, chair of the Royal College of General Practitioners, said there needs to be clear messaging to the public on what is a “necessary condition to look after” while GPs focus on the Covid-19 booster rollout.
Mr Raab did not rule out further measures, beyond working from home advice and vaccine passports, being considered for Christmas or New Year. He told Times Radio: “These issues are always discussed but we have got Plan B, that’s what we think is required over the Christmas period.”
CBI boss Tony Danker said the Government campaign to urge people to get their booster jab has had a “chilling effect” on certain sectors of the economy. He told BBC Radio 4’s Today Programme: “People should be worried enough to go and get a booster urgently, but not so worried to stop going to shops, restaurants or airports.”.
Sir John Bell said symptoms of Omicron were often sore throat, aching muscles, particularly around the back, stuffy nose, some stomach upset and loose stools.
The number of confirmed coronavirus cases in the capital spiralled to 11,791 on Monday, taking the total in the last four days to 40,275.
Mr Javid revealed on Monday that ten people were in hospital with Omicron, with one individual having died with the variant.
Most of these ten had received two vaccines and range in age from 18 to 85, though there are no details on whether they have underlying conditions.
Ministers have pledged to offer a booster to all adults by the end of the month.
There have been 6,213,113 first dose Covid vaccinations administered in London so far, 5,630,013 second dose innoculations, and 2,311,984 boosters or third doses for individuals vulnerable to the virus.lnerable to the virus.