A new strain of coronavirus has been identified southern England as tougher restrictions were imposed in London.
The capital and parts of Essex and Hertfordshire will face Tier 3 restrictions from Wednesday following "very sharp, exponential rises" in cases.
Health Secretary Matt Hancock told MPs the number of cases involving the new variant was "increasing rapidly".
"Initial analysis suggests that this variant is growing faster than the existing variants," he said.
"We've currently identified over 1,000 cases with this variant predominantly in the South of England although cases have been identified in nearly 60 different local authority areas."
The Health Secretary said there was nothing to suggest the variant was more likely to cause serious disease and he suggested the vaccine should still be effective.
"But it shows we've got to be vigilant and follow the rules and everyone needs to take personal responsibility not to spread this virus," he said.
