Video report by ITV News Political Editor Robert Peston
Hosting a press briefing in response to the prime minister's new Covid restrictions tier system for England, Sir Keir heaped pressure on Boris Johnson calling for him to "act now and break the cycle" as infection rates continue to rise.
The Labour leader said a short "circuit break" period of restrictions should include closing non-essential offices, limiting household mixing to one household, and closing all pubs, bars and restaurants. Schools, however, could remain open.
"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 warned.
Robert Peston believes that if the latest Covid tier measures are not ‘bearing down’ on coronavirus cases in the next two weeks, we could be in for ‘some kind of national lockdown’ which could be similar to that imposed on the UK in March and April, with the exception of schools remaining open:
It comes as Boris Johnson's government has been accused of ignoring its own scientists' calls for such measures.
Newly released documents show the Scientific Advisory Group for Emergencies (Sage) told the government - at a meeting on September 21 - that a "circuit breaker" lockdown in England would help slow the spread of the virus.
Sir Keir said a "circuit breaker" would provide an opportunity to "reset" and "rectify some of the mistakes the government has made."
He took aim at the current track and trace system: "In particular, to get a grip on testing and hand over track and trace to local authorities."
Watch the full statement from Labour leader Sir Keir Starmer:
The Labour leader referenced the Sage documents that suggested such a measure nearly three weeks ago.
"You know that the science backs this approach. You know that the restrictions you’re introducing won’t be enough," he said.
"You know that a circuit break is needed now to get this virus under control.
"You can’t keep delaying this and come back to the House of Commons every few weeks with another plan that won’t work.
Sir Keir suggested the PM would have Labour's support in the Commons were he to put the measure to a vote.
Under the proposed measure, the Labour leader said schools could remain open and suggested the "circuit break" could be enforced over half-term to "minimise disruption".
He added: "A circuit break would require significant sacrifices across the country.
"It would mean only essential work and travel. That everyone who can work from home should do so."
Quizzed on the economic damage to the economy of a "circuit break" lockdown, the Labour leader told reporters: "The damage to the economy will be caused by weeks and months of the approach the government has set out".
He said the measure would need "to be accompanied by extensive support for jobs, businesses and our local economies."
Adding: "If we’re requiring businesses to close, we must provide the financial support necessary to protect people and our local communities – because every job matters and every business matters."
MPs have now approved the new local Covid alert levels during a vote on Tuesday - with the restrictions likely to be enforced from Wednesday.
Responding to the Labour leader's most dramatic intervention to date, Conservative health minister Nadine Dorries tweeted: "You need to speak to your Manchester Mayor (Andy Burnham) who has been opposed to intervention measures all the way!"
There has, however, been criticism from senior Conservatives over the tier plan.
Sir Bernard Jenkin said the local advice in his Harwich and North Essex constituency was that a "short, sharp, shock" would be the best way of tackling the pandemic.
The chairman of the Commons’ Liaison Committee told Times Radio: "If we nipped it hard now, we would be well back to manageable levels before Christmas. If we leave it for another five or six weeks, then … the tail of the bell curve goes well into January."