Keir Starmer said Labour would limit spending to address the “damaged economy and country” inherited from the Conservatives if his party won the UK's next general election.
In his first speech of the new year, the UK opposition leader said reforms would be needed to get the country back on track.
Taking a question on his pledge not to borrow heavily, Mr Starmer said: “That commitment in relation to the chequebook is because we know we are going to inherit a badly damaged economy and badly damaged country.
“Therefore we have to be absolutely clear that we can’t just spend our way out of that mess.
“Obviously we will set out our case as we go into the election and we have already set out our fiscal rules in terms of spending, only borrowing to invest, and getting debt down as a percentage of our economy.
“After 13 years of failure on every level, we know we are going to inherit a very badly damaged economy, and that’s what drives me to say we won’t be getting out that big government chequebook.
“Everything we say we will do will be fully costed and set out, as it already has been, and we’ll do that going into the election.”
He accused UK Prime Minister Rishi Sunak of offering the country only "more promises, more platitudes" criticised "sticking plaster politics".
Mr Starmer said the prime minister was in denial about the problems facing the country.
Flanked by party members, Mr Starmer told the audience in Stratford that his promises "should not be taken as code for Labour getting its big government chequebook out again".
"Of course, investment is required — I can see the damage the Tories have done to our public services as plainly as anyone else," he told the audience.
"But we won't be able to spend our way out of their mess — it's not as simple as that.
"There is no substitute for a robust, private sector, creating wealth in every community."
His first speech of the year comes as the government tackles a crisis in the National Health Service and continuing strike action, as households struggle with cost-of-living challenges.
Mr Sunak was accused of being light on detail in his own speech in Stratford, in which he promised to halve inflation, deal with NHS waiting lists and tackle small boat crossings in the Channel.
Mr Starmer told his audience that Mr Sunak offered "commentary without solutions".
"More promises, more platitudes. No ambition to take us forward. No sense of what the country needs. Thirteen years of nothing but sticking plaster politics," he said.
The Labour leader, whose party would abolish and replace the House of Lords if it wins the next election and replace it with a regionally elected chamber, said the country needed a new approach to politics and the economy.
"This year, let's imagine instead, what we can achieve if we match the ambition of the British people," he said.
"That's why I say Britain needs a completely new way of governing.
"You can't overstate how much a short-term mindset dominates Westminster, and, from there, how it infects all the institutions which try, and fail, to run Britain from the centre."
He said the country faced "sticking plaster politics". "The long-term cure, that always eludes us," he said.
On the NHS, he said the crisis had been "an iceberg on the horizon".
The new approach to governing, he said, will be driven by "national missions", which the party will set out in the coming weeks.
His party, he said, had changed under his leadership.
"People know we care, they always know the Labour Party cares and they can now see a party that is both competent and compassionate," he said.
Introducing her party leader on Thursday, the shadow chancellor Rachel Reeves asked the audience: "Are you and your family better off than you were 13 years ago?
"Does anything in Britain work today better than it did 13 years ago?"
She said the country faced the choice of following the path of "managed decline" under the Conservatives, or "we can change course, to build a greener, fairer, and more dynamic Britain".
