The Labour leader heaped pressure on to the Prime Minister on Tuesday after it emerged he dismissed a recommendation for the measure from Government scientists three weeks ago.
In his most dramatic intervention to date, Sir Keir said a two to three-week national lockdown was needed to improve test and trace and prevent a “sleepwalk into a long and bleak winter”.
He told a televised press conference that Mr Johnson was “no longer following the scientific advice” by proposing “far less stringent restrictions” than suggested by the Scientific Advisory Group for Emergencies (Sage).
Sir Keir said: “There’s no longer time to give the Prime Minister the benefit of the doubt. The Government’s plan simply isn’t working. Another course is needed.”
He said schools must stay open but that all pubs, bars and restaurants should be closed during the circuit-breaker, while firms are compensated so “no business loses out” in order to “break the cycle” of infection.
“If we don’t, we could sleepwalk into a long and bleak winter. That choice is now for the Prime Minister to make. I urge him to do so,” Sir Keir said.