A "new variant" of coronavirus has been identified in the UK, which is believed to be causing the faster spread in the southeast, British Health Secretary Matt Hancock said.
Over 1,000 cases of the new variant have been identified, "predominantly in the south of England", Hancock told the House of Commons this afternoon.
The new variant is spreading faster than the existing strain of coronavirus and is believed to be fueling the "very sharp, exponential rises" in cases across the southeast part of the commonwealth, he said.
Hancock told the Commons that scientific experts do not yet know whether the increases are the result of the new variant, but added: "No matter its cause we have to take swift and decisive action which unfortunately is absolutely essential to control this deadly disease while the vaccine is rolled out."
A decision on whether the capital and parts of the southeast should move up a tier was due on Wednesday, but Hancock said the surge in cases made it necessary to bring the changes forward, with London now set to enter Tier 3 on Wednesday.
Prof Alan McNally, an expert at the University of Birmingham, said UK testing labs had picked up on this new variant in the last few weeks. He told the BBC: "Let's not be hysterical. It doesn't mean it's more transmissible or more infectious or dangerous.
Scottish First Minister Nicola Sturgeon said the new COVID variant was a concerning development.