Jeremy Corbyn has pledged to stand up for small businesses against "cheating" corporate titans who delay payments to suppliers and gain from sweetheart tax deals.
The Labour leader said late payments were forcing more than 50,000 small firms out of business every year in what he described as a "national scandal".
Mr Corbyn said people running small firms were "fed up with there being one rule for them and another for big business".
"Whether it's tax avoidance, late payments for suppliers or business rates, giant companies seem to be privileged and sometimes a law unto themselves."
The Labour leader set out his approach at a speech to the Federation of Small Businesses (FSB) - which welcomed some of his comments but expressed worries about plans to hike the National Living Wage to £10 an hour.
At the event in London, Mr Corbyn said big firms were withholding more than £26bn a year from small suppliers by paying their bills late, according to Labour's analysis of figures from payments organisation Bacs.
Mr Corbyn said: "Big companies are managing their cash by borrowing - interest free - from their suppliers.
"It's a national scandal. And it's stopping businesses from growing and causing thousands to go bust every year. It kills jobs and holds back economic growth."
Labour plans to require companies bidding for public sector contracts to pay suppliers within 30 days and would consider fines for persistent offenders in the private sector.
Mr Corbyn also outlined plans to raise corporation tax - but only for bigger companies - to pay for investment in skills.
He added that most "play by the rules" on tax and recognise that paying it is "essential to a civilised society" - but some big firms do not.
"They use all sorts of elaborate ruses like shell companies or tax havens, or offshore trusts to sidestep the rules.
"When we say we'll clamp down hard on tax avoidance and legislate to close loopholes, it's not anti-business it's anti-cheating."
FSB national chairman Mike Cherry welcomed Mr Corbyn's proposals on the "late payments crisis" facing small businesses.
But he said that firms were already struggling with the latest increase in minimum wage levels and many would see another increase now as "unaffordable".
Conservative MP Amanda Milling said: "Despite what Labour are saying today, all that Jeremy Corbyn can offer small businesses across Britain are higher corporation tax, an increased deficit and more economic instability."