Boris Johnson defended his decision to implement the new lockdown at the time he did, saying the new, more contagious, coronavirus variant offered little choice.
Responding to criticism over his decision to close primary schools a day after he told them they should open, Johnson said he did "everything in our power to keep them open" until "every other option had been closed off".
"And as was the case last spring, our emergence from the lockdown cocoon will be not a big bang but a gradual unwrapping." he said.
Britain has been among the countries worst-hit by COVID-19, with the highest death toll in Europe. With the appearance of the new variant, it has seen case numbers repeatedly reach record highs, stretching the country's health service.
He said the legislation would run until March 31 "not because we expect the full national lockdown to continue until then, but to allow a steady, controlled and evidence-led move down through the tiers on a regional basis".