Prime Minister Boris Johnson ordered England back into a national lockdown after the United Kingdom passed the milestone of one million Covid-19 cases and a second wave of infections threatened to overwhelm the health service.

The UK, which has the biggest official death toll in Europe from Covid-19, is grappling with more than 20,000 new coronavirus cases a day and scientists have warned the “worst case” scenario of 80,000 dead could be exceeded.

Johnson, at a hastily convened news conference in Downing Street after news of a lockdown leaked to local media, said that...