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 said it was “almost certain” that hundreds of thousands of people were being infected with the variant every day.

Prime Minister Boris Johnson has faced a rebellion in his governing Conservative Party over the measures he has taken so far to try to curb the latest COVID-19 wave.

“Currently observed numbers of Omicron infections admitted to hospital in the UK are probably around one tenth of the true number because the data lags hospital reporting,” the advisors said in minutes of a meeting on Dec. 16.

It was too early to reliably assess the severity of disease caused by the Omicron variant, but 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.

Last January, before Britain’s vaccination campaign gathered speed, daily hospital admissions in the United Kingdom as a whole surged above 4,000.

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.