The UK government has introduced tough new measures for some areas of England in response to the fast spread of a new strain of the COVID-19 virus. 

As of Sunday morning, the country's capital London as well as parts of southeastern England will enter a new Tier 4, with restrictions likened to the first nationwide lockdown earlier this year. 

The new strain "could increase the R [rate of infection] by 0.4 or more and is up to 70 percent more transmissible than the original disease," Prime Minister Boris Johnson said, who warned that it was "spreading very fast."

The UK's chief scientific officer Patrick Vallance said the new strain first occurred in mid-September in London or Kent. He said scientists had detected 23 changes in the new variant, mainly in the way it binds to human cells – at "an unusually large number" – and was responsible for "a significant and substantial" increase in the number of cases in areas where it was circulating.

It now accounts for 62 percent of those who have tested positive for the virus in London, while the number is above 45 percent in other regions put under new restrictions. 

"There is no evidence at the moment to suggest the new variant causes more severe disease," Vallance added. 

"Nothing has been seen so far to suggest it alters the immune response and the vaccine response should be adequate for this virus," he added.

Prime Minister Boris Johnson said he had "a very heavy heart" when announcing a reversal of his existing plans to relax rules on household mixing over Christmas.

In the new Tier 4 areas, residents must stay at home, non-essential shops are to close, and people are being urged to work from home if they can, with travel to and from Tier 4 areas banned, meaning many Christmas plans are left in tatters. 

In all other tiers, three households will only be allowed to mix on December 25 and not the five days the government had previously announced.