LONDON — Britain reported 78,610 new coronavirus cases on Wednesday, the highest number of infections in a single day since the start of the pandemic, and stark confirmation that the Omicron variant is rampaging across the country.

New cases spiked by a third since Tuesday; the number is more than 10,000 higher than the previous worst day for infections, Jan. 8, when the Alpha variant was ravaging the country. The seven-day average of new cases is 65,008, a 19.1 percent increase over the previous seven-day period. Officials didn’t specify what share of the new cases might be Omicron, though they said a majority in London were from the variant.

Chris Whitty, the chief medical officer for England, warned that further records would be broken in coming days, with the Omicron variant doubling at a rate of less than every two days in parts of the country. While the effect on hospitalization and mortality rates remains unclear, he warned that Britain’s National Health Service would face a deluge of patients simply because the growth in cases was so explosive.

“This is a really serious threat,” Dr. Whitty said at a news conference, alongside Prime Minister Boris Johnson and the medical director of primary care for N.H.S. England, Nikki Kanani. “It is moving at an absolutely phenomenal pace.”

Mr. Johnson redoubled his campaign for people to get vaccine booster shots. About 650,000 people received shots on Tuesday, another record-breaking day. Mr. Johnson has set a goal of delivering boosters to all adults by the end of the month, a target that would require administering more than 1 million shots a day.

While Mr. Johnson did not announce any additional restrictions on Wednesday, he urged the public to be judicious in socializing during the holidays. Parliament on Tuesday passed the government’s plan to impose a system of vaccine certification to enter nightclubs and large indoor venues, though nearly 100 members of Mr. Johnson’s Conservative Party voted against the measure.

“We’re not canceling people’s parties,” Mr. Johnson said. “What we are saying is, think carefully before you go.”

The prime minister has been under fierce political pressure in recent weeks after reports that his staff held holiday gatherings at Downing Street last year, at a time when the government was instructing people not to meet with friends or even family members. A report on those allegations is expected to be released in coming days, and Mr. Johnson said he welcomed the investigation.

While there is preliminary evidence from South Africa that the Omicron variant is less severe than previous variants, Dr. Whitty cautioned against over-interpreting the data.

In Britain, 774 people were admitted to hospitals on Wednesday, a 10.4 percent increase over the last seven-day period, while 165 people died, a 5 percent decline over the seven previous days.

Omicron’s spread has been particularly dramatic in London, where the vaccination rate is lower than other parts of the country. The prime minister said hospitalization rates in London were up by a third.

“We’ve got two epidemics on top of each other,” Dr. Whitty said, “a flat Delta epidemic and a rapidly growing Omicron epidemic.”