"While hospitalisations currently remain broadly flat, the rapid spread of Delta would likely "lead to a substantial resurgence of hospitalisations" and put pressure on the NHS," said UK Health Minister Matt Hancock
The variant is "becoming the dominant strain in some parts of the country" such as Bolton and Blackburn, Xinhua news agency quoted hancock as saying to Sky News on Sunday.
An estimated 16.4 million people living in the new Tier Four areas, which include London and much of the surrounding region, will have to stay at home for a fortnight.