Johnson said a new, more contagious variant of the coronavirus was spreading at great speed and urgent action was needed to slow it down.
"As I speak to you tonight, our hospitals are under more pressure from COVID than any time since the start of the pandemic," Johnson said Monday (January 4) in a televised address to the country.
"With most of the country already under extreme measures, it's clear that we need to do more together to bring this new variant under control... We must therefore go into a national lockdown, which is tough enough to contain this variant. That means the government is once again instructing you to stay at home."
Johnson said the measures would include school closures from Tuesday and rules requiring most people to stay at home apart from essential shopping, exercise and other limited exceptions.