Rapid transmission of a newly-discovered variant of Covid-19 in London and nearby areas prompted UK Prime Minister Boris Johnson on Saturday to put the capital under similar restrictions that were imposed under the national lockdown in November, while relaxations allowed for Christmas have been cancelled.

The new variant, he said, was no less lethal than the original version and there is no evidence that a vaccine is less effective against it; but he added that there is much that is not known about it.

The World Health Organization has been informed of the development.

Johnson announced the creation of a new tier – Tier 4 – in the Covid-19 alert system, under which towns and areas in England are placed. London has been in the top Tier 3 with the toughest restrictions, but has gone further in the lockdown-like Tier 4.

The curbs take effect from Sunday morning until December 30.

Johnson said in a televised briefing from Downing Street, “There is still much we don’t know. While we are fairly certain the variant is transmitted more quickly, there is no evidence to suggest that it is more lethal or causes more severe illness. Equally there is no evidence to suggest the vaccine will be any less effective against the new variant.

“Our experts will continue their work to improve our understanding of the variant. So we are learning more about this variant as we go. But we know enough already to be sure that we must act now.

“First, we will introduce new restrictions in the most affected areas – specifically those parts of London, the South East and the East of England which are currently in tier 3. These areas will enter a new tier 4, which will be broadly equivalent to the national restrictions which were in place in England in November.”

Life under Tier 4 in London and southeast England from Sunday morning will be similar to the lockdown, which includes people staying at home, unless moving out for exemptions allowed; non-retail shops, gyms and leisure facilities to close; work from home; people must not travel out of Tier 4 areas.

Health secretary announced the discovery of the variant in parliament on Monday, while Chris Whitty, chief medical officer for England, gave more details of the variant identified by Public Health England’s genomic surveillance. According to Whitty, this is a “critical and dangerous moment”.

Whitty added, “As a result of the rapid spread of the new variant, preliminary modelling data and rapidly rising incidence rates in the southeast, the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) now considers that the new strain can spread more quickly.

“We have alerted the WHO and are continuing to analyse the available data to improve our understanding. There is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments, although urgent work is underway to confirm this.”

Johnson said Christmas this year “will be very different”, adding that people in all tiers should not travel, which will affect travel to meet families over the festive period.

The curbs come days after he had announced relaxations for Christmas, which are now scrapped.

“As prime minister, it is my duty to take the difficult decisions, to do what is right to protect the people of this country. Given the early evidence we have on this new variant of the virus, and the potential risk it poses, it is with a heavy heart that I must tell you we cannot continue with Christmas as planned,” Johnson said.