Sir Keir Starmer has confirmed there will be tax rises in the budget to prevent a "devastating return to austerity" and rebuild public services.
In a speech in Birmingham, the prime minister also announced the £2 bus fare cap will rise to £3, while £240m will be injected into efforts to get people back to work.
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It follows weeks of speculation on how the government plans to grow the economy and close a £40bn blackhole in the nation's finances when it delivers its first fiscal statement on Wednesday.
Giving a teaser of what is to come, Sir Keir said: "It is time to embrace the harsh light of fiscal reality.
"Stability to prevent chaos. Borrowing will drive long term growth. Tax rises to prevent austerity and rebuild public services."
Sir Keir did not specify what tax rises would be included, but it is widely expected that employer national insurance will go up, alongside possible increases to capital gains and inheritance tax.
Ministers have been facing repeated questions about the government's definition of "working people", after Labour's election manifesto pledged not to increase taxes on working people - explicitly ruling out a rise in VAT, national insurance and income tax.
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What is likely to be in the budget?
PM denies 'waging war against middle Britain'
Facing down critics, the prime minister said a return to austerity would be "devastating" and "disastrous".
"This is not 1997 when the economy was decent, but public services were on their knees.
"And it's not 2010 where public services were strong, but the public finances were weak.
"We have to deal with both sides of that coin. These are unprecedented circumstances."
Sir Keir said the budget, to be delivered by Chancellor Rachel Reeves, will include funding for local services to help get people back to work.
He said the UK is "the only G7 country for whom economic inactivity is still higher than it was before COVID".
"That's not just bad for our economy, it's also bad for all those who are locked out of opportunity," he said.
"So the chancellor will announce £240m in funding to provide local services that can help people back into work."
Clearest indication yet that tax rises are coming
Liz Bates
Political correspondent
@wizbatesThis may be the starkest terms we've ever heard the prime minister speak in as Wednesday's defining budget looms.
He repeated the refrain of "tough choices" suggesting the pain inflicted by the cuts to the winter fuel allowance was just the beginning.
This is also the clearest expression ever that tax rises are coming, with Sir Keir Starmer describing them as necessary to prevent austerity.
What was also clear was his concept of "working people" - those who are just about getting by financially, and his determination to put them first.
It was resolute and determined, but as the budget approaches it does create a political risk, as those who don't fit into that narrow category - business owners, pensioners, and others - are left wondering how hard they are going to be hit.
Taking questions afterwards, he confirmed reports that the £2 bus fare cap will go up by 50%.
He said the previous government only funded the current limit to the end of 2024 "and therefore that is the end of the funding in relation to a £2 capped fare".
"I do know how much this matters, particularly in rural communities where there's heavy reliance on buses," he said.
"And that's why I'm able to say to you this morning that in the budget we will announce there will be a £3 cap on bus fares to the end of 2025 because I know how important it is."
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