Volunteers paint red hearts representing the victims who died of COVID-19 on the National COVID Memorial Wall outside St Thomas' Hospital in London, Britain, on March 31, 2021.(Photo: Xinhua)

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, "modeling indicates a peak of at least 3,000 hospital admissions per day in England," they said in minutes of a meeting on Thursday.

In January 2020, before Britain's vaccination campaign gathered speed, daily hospital admissions in the UK 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" as COVID-19 hospital admissions in the city rose by nearly 30 percent this week.

He said health worker absences had also increased.

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 percent 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.

Reuters