The newly appointed leader tentatively hinted at an increase in taxes when pressed about what Labour would do to pay for the extra costs associated with the Covid-19 response.
But he insisted ‘pretending’ he could answer what would be needed in 2024 is ‘not something I'm going to do’, acknowledging the damage to the economy is unclear.
Sir Keir added that a different approach to the one adopted by the Conservatives since 2010 will be needed, with key workers requiring a better support arrangement.
The government has committed tens of billions of pounds of taxpayers' cash in a bid to support businesses and keep people in jobs during the lockdown.
Sir Keir said the coronavirus would frame the economic debate and how people are valued.
He said: “We can't go out every Thursday on our doorsteps and clap for our key workers who are keeping us going in this period, and then at the end of this return to business as usual.”
Sir Keir said Labour needed to ‘spell out that vision of a better society’ once the outbreak has been dealt with.
He warned austerity had caused ‘great harm’ since 2010 when told the policy could continue in some form to pay off spending connected to Covid-19.
He added: “What we can't have is another decade of austerity on top of this decade of austerity.
“Just look what it's done to the health service, public services, social care, all of these areas that are now so being relied on have been cut for 10 years.”
On what Labour's approach would be, Sir Keir said: “We may well have to raise taxes, of course, but you're asking me a question that nobody can realistically answer because we don't yet know what the damage to the economy is going to be.”
“But it is clear we're going to have to take a different approach.
“Lots of people who are key workers today...have been under-paid and under-valued for years. That cannot go on. There's got to be a fundamental difference going forward.”