London is to move into the highest level of coronavirus restrictions due to concern about spiralling numbers of infections, the health minister announced Monday.

The British capital's move into "Tier 3" from 0001 GMT Wednesday means theatres as well as pubs, restaurants and other hospitality venues will have to close, except for takeaway food. 

People are not supposed to socialise with anyone not from their household, but they can meet in groups of up to six in public places outside.

Health Secretary Matt Hancock said London had seen a "sharp rise" in daily cases and hospital admissions, and there was public health concern about a new strain of the coronavirus.

"This action is absolutely essential, not just to keep people safe but because we have seen early action can prevent more damage and longer-term problems later," he told parliament.

In some areas, cases are doubling every seven days, he said, warning: "It only takes a few doublings for the NHS (National Health Service) to be overwhelmed."

Scientists have identified a "new variant" of the virus in the south of England that may be causing infections to spread faster, Hancock added.

But he said: "There's currently nothing to suggest it is more likely to cause serious cases and the latest clinical advice is it is highly unlikely to fail to respond to a vaccine."

Currently London is in "Tier 2", which means non-essential shops and services can open, but it currently has one of the highest infection rates in the country.

Tier 3 or "very high alert" means that while bars, pubs and cafes can only offer takeaway food and drinks, shops and hairdressers can stay open, as can schools. 

However several London districts are sending children home from Tuesday as a precaution, after the government said last week it was concerned at the rising numbers of cases among secondary school-age children.

Testing of 11 to 18-year-old students began in the worst-affected areas on Monday.

The switch to stricter measures is a fresh blow to businesses already hit by repeated restrictions this year and in the run-up to Christmas, normally their busiest time.

Parts of the counties of Essex, Kent and Hertfordshire, which border London, will also be placed in Tier 3.

London Mayor Sadiq Khan called the rise in cases "deeply concerning" but warned Prime Minister Boris Johnson the move would be "catastrophic" for pubs, bars and arts venues.

On Friday, the "R number" in London indicating how many other people someone with the virus will infect was between 0.9 and 1.1, according to the government's website.

More than 201,000 cases have been registered in the capital and more than 7,000 people have died in hospital.