* Number of cases of Omicron coronavirus variant jump
* London mayor declares "major incident" to help hospitals
* Government scientific advisors say many cases unreported
* Advisors say more action needed to prevent hospitalisation surge
* Johnson faced anger from his own lawmakers to existing measures
Britain reported a surge in cases of the Omicron coronavirus variant on Saturday which government advisors said could be just the tip of the iceberg, and London's mayor declared a "major incident" to help the city's hospitals cope.
The number of Omicron cases recorded across the country hit almost 25,000 as of 1800 GMT on Friday, up by more than 10,000 cases from 24 hours earlier, the UK Health Security Agency (UKHSA) said.
Seven people believed to have had the Omicron variant had died as of Thursday, up from one death in the UKHSA's previous data which ran up to Tuesday. Admissions to hospital of people thought to have the variant increased to 85 from 65.
The government's Scientific Advisory Group for Emergencies (SAGE) said it was "almost certain" that hundreds of thousands of people were being infected with the variant every day and were not being picked up in the figures.
SAGE said without a further tightening of Covid-19 rules, "modelling indicates a peak of at least 3,000 hospital admissions per day in England," they said in minutes of a meeting on Dec. 16.
Last January, before Britain's vaccination campaign gathered speed, daily hospital admissions in the United Kingdom as a whole surged above 4,000.
Prime Minister Boris Johnson has faced a rebellion in his governing Conservative Party over some of the measures he has taken so far to try to curb Covid-19's latest spread. A newspaper said on Saturday that Johnson's Brexit minister, David Frost, had resigned in part because of the new rules.
The advisors said it was too early to assess the severity of disease caused by Omicron but if there was a modest reduction compared to the Delta variant, "very high numbers of infections would still lead to significant pressure on hospitals".
London Mayor Sadiq Khan declared a "major incident" - which allows for closer coordination between public agencies and possibly more central government support - as Covid-19 hospital admissions in the city rose by nearly 30% this week.
He said health worker absences had also increased.
"This is a statement of how serious things are," he said.
Khan, from the opposition Labour Party, also declared a major incident in January, when rising Covid-19 cases threatened to overwhelm hospitals.
The Omicron variant is estimated to account for more than 80% of new Covid-19 cases in London, officials said on Friday.
Johnson was due to chair an emergency committee meeting over the weekend with the devolved administrations in Scotland, Wales and Northern Ireland, which have their own powers over public health.
A report in The Times newspaper said officials were preparing draft rules which, if introduced, would ban indoor mixing in England -- except for work -- for two weeks after Christmas when pubs and restaurants would be limited to outdoor table service.
People would be able to meet in groups of up to six outdoors, the newspaper said, adding that ministers were yet to formally consider the plans.
Johnson said on Friday "we are not closing things down".
A government spokesperson said the government would continue to "look closely at all the emerging data and we'll keep our measures under review as we learn more about this variant".
The number of all new Covid-19 cases reported in official data fell to 90,418 from a record high of more than 93,000 on Friday, but that was still the country's second-highest daily toll. Figures typically dip at the weekend.
Cases were up 44.4% over the seven days to Dec. 18 compared with the previous week.
Police clashed with a group of protesters opposed to the latest Covid-19 restrictions near Johnson's Downing Street office and residence on Saturday. A number of officers were injured but so far no arrests had been made, police said.