Labour would go through every Covid contract to claw back cash from dodgy suppliers who received Government cash during the pandemic.
Shadow Chancellor Rachel Reeves put firms on notice that "we want our money back" if they failed to deliver during the coronavirus crisis.
In a fiery speech to Labour conference, she vowed to clean up after the Government, which has faced repeated criticism over how lucrative Covid contracts were handed out at the height of the pandemic.
She also made a major announcement of an "additional £28bn of capital investment in our country's green transition for each and every year of this decade".
Ms Reeves declared: "I will be Britain's first green Chancellor."
She added: "I will not shirk our responsibility to future generations and to workers and businesses in Britain.
“No dither, no delay. Labour will meet the challenge head on and seize the opportunities of the green transition."
The cash would go towards areas such as jobs in green industries, giga-factories to build batteries for electric vehicles, building offshore wind turbines in Britain, planting trees and building flood defences.
Delegates gave Ms Reeves a warm reception for her first major conference speech, where she set out plans to scrap business rates and increase the digital service tax to 12% to help struggling high streets.
She pledged to overhaul the tax system - and vowed not to "balance the books on the backs of working people".
"We will look at every single tax break and if it doesn't deliver for the economy or the taxpayer, then we will scrap it," she told delegates.
She restated Labour's commitment to closing a loophole that gives charitable status for private schools - and diverting the money into state education.
Ms Reeves blasted the "out of touch" Tories for pressing ahead with the £20-a-week Universal Credit cut as cost of living pressures mount for families.
She said: "With inflation rocketing, people are feeling the squeeze at the supermarket checkout, at the petrol pump and when their energy bills arrive as little luxuries feel further out of reach."
She attacked the Government for delivering an "outsourcing bonanza" during the pandemic, and accused ministers of handing over billions of pounds in crony contracts.
In a furious attack, she said millions had been handed to businesses that produced face masks that couldn't be used or Covid test kits that had to be recalled.
She added: "I say today: To those who have secured Covid contracts and have not delivered. I give you notice. We expect that money back.
"We will set up a team to go through every line of every failed contract where value was not delivered, and claw back every penny of taxpayers' money we possibly can.
"Because that money belongs in our police. It belongs in our schools and it belongs in our NHS."
