LONDON:Britain said on Monday that the Omicron variant of coronavirus was spreading at a "phenomenal rate" and accounted for about 40% of all infections in London so people should get an additional dose of the vaccine because double-jabbed people are vulnerable.

Since the first cases of the Omicron variant were detected on Nov. 27 in the United Kingdom, Prime Minister Boris Johnson has imposed tougher restrictions and told the nation on Sunday that a "tidal wave" of Omicron was about to hit.

Britain says that unless action is taken there could be a million people infected with Omicron, which scientists say can still infect people who are double-vaccinated, by the end of the month.

"What we now know about Omicron is that... it's spreading at a phenomenal rate, something that we've never seen before, it's doubling every two to three days in infections," Health Secretary Sajid Javid told Sky News.

"That means we're facing a tidal wave of infection, we're once again in a race between the vaccine and the virus."

The pound fell 0.4% to $1.3225, while it was broadly steady against the euro at 85.29 pence.

Johnson, who is grappling with a rebellion in his party over measures to curb the Omicron spread and an outcry over alleged Downing Street parties during last year's lockdowns, said people should rush to get booster vaccines to protect "our freedoms and our way of life".

The novel coronavirus, which was first detected in China in late 2019, has killed 5.3 million people, wiped out trillions of dollars in economic output and turned normal life upside down for people across the world.

Data released on Friday showed that vaccine efficacy against symptomatic infection was substantially reduced against Omicron with just two doses, but a third dose boosted protection up to over 70%.

Though Javid said there had been no deaths yet confirmed in England and just 10 people were hospitalised in England with the variant, he said Omicron was probably behind around 40% of infections in London.

He said that while symptoms of the variant might be milder, its swift spread meant that unless the government acted then the health service could be overwhelmed.

"Even when a virus is mild, a small percentage of people from a very large number still can equal a high number of hospitalizations," Javid said.

"Two doses are not enough, but three doses still provide excellent protection against symptomatic infection," he said.