Brexit could give Whitehall and councils more power to support British firms bidding for public sector contracts, Jeremy Corbyn will say today.
The Labour leader will say that while leaving the European Union "presents many challenges" for Britain, it could also allow changes to procurement rules in order to favour local suppliers and jobs.
Mr Corbyn will also demand sweeping changes to the way almost £200 billion is spent in the private sector, with requirements for firms bidding for contracts to recognise trade unions and curb boardroom excess by moving towards a 20-1 pay ratio between high and low earners.
The Labour leader will say: "While Brexit presents many challenges to Britain, it can give us more powers to encourage best practices and support new and existing businesses and industries in Britain.
"While the Conservatives seem intent on using Brexit to turn us into a low-wage tax haven, Labour will use every power possible to upgrade our economy so we can all lead richer lives."
He will insist that a Labour government would not use taxpayers' money to "subsidise bad corporate behaviour".
In a speech in Doncaster, Mr Corbyn will say: "For years we've been told that there's nothing that can stop the race to the bottom in the jobs market that is making people's lives harder and holding back our economy.
"Well, today I say, Britain doesn't have to be so meek, and settle for things getting worse and more insecure for so many. We can make the change we need if we understand the power we already have - and how we can better use it.
"National and local government spends £200 billion per year in the private sector. That's an incredible purchasing power, which we can use to support the good companies and improve the behaviour of the bad ones that undercut with unfair practices.
"Under the next Labour government, Britain will subsidise bad corporate behaviour no longer. Our business partners should have the same values we as a country hold: enterprise, fairness, high-quality service and doing right by everyone."
Firms bidding for contracts would need to comply with collective bargaining agreements, pay suppliers within 30 days, maintain high environmental standards, provide training and apprenticeship opportunities, comply fully with tax requirements and adopt "best practices" in equal opportunities.
All of those measures are possible within existing European Union rules, Labour said.
After Brexit, central and local government will be outside the EU's strict procurement rules and Labour would introduce "local jobs and content requirements" for contracts that fall below World Trade Organisation (WTO) thresholds.
The party also called on the Government to conduct an impact assessment of the WTO general procurement agreement (GPA) coverage on local authorities in the UK.
Currently the GPA thresholds are set at almost £5.5 million for construction contracts, £141,621 for central government goods and services and £217,878 for sub-central government contracts.